diff --git a/CHANGELOG.md b/CHANGELOG.md
index 18d63a1..a2bd260 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,18 @@
 # Changelog
 
-### [0.5.1](https://github.com/googleapis/python-dataflow-client/compare/v0.5.0...v0.5.1) (2022-05-05)
+## [0.5.2](https://github.com/googleapis/python-dataflow-client/compare/v0.5.1...v0.5.2) (2022-06-03)
+
+
+### Bug Fixes
+
+* **deps:** require protobuf <4.0.0dev ([#106](https://github.com/googleapis/python-dataflow-client/issues/106)) ([3170fe7](https://github.com/googleapis/python-dataflow-client/commit/3170fe7af4b3de2802251b2f24ca73d2060d560b))
+
+
+### Documentation
+
+* fix changelog header to consistent size ([#107](https://github.com/googleapis/python-dataflow-client/issues/107)) ([df80541](https://github.com/googleapis/python-dataflow-client/commit/df805416bd144da9d2c0e4c49a7d478580237b28))
+
+## [0.5.1](https://github.com/googleapis/python-dataflow-client/compare/v0.5.0...v0.5.1) (2022-05-05)
 
 
 ### Documentation
@@ -14,7 +26,7 @@
 
 * Add capabilities field to SdkHarnessContainerImage ([#84](https://github.com/googleapis/python-dataflow-client/issues/84)) ([3d86713](https://github.com/googleapis/python-dataflow-client/commit/3d867138d9abc434f83556a0177a89b23e187555))
 
-### [0.4.1](https://github.com/googleapis/python-dataflow-client/compare/v0.4.0...v0.4.1) (2022-03-05)
+## [0.4.1](https://github.com/googleapis/python-dataflow-client/compare/v0.4.0...v0.4.1) (2022-03-05)
 
 
 ### Bug Fixes
@@ -39,7 +51,7 @@
 
 * remove typo in docstring ([#72](https://github.com/googleapis/python-dataflow-client/issues/72)) ([db91cc2](https://github.com/googleapis/python-dataflow-client/commit/db91cc2c65cf43f766385c0f973dd58c4233c9c7))
 
-### [0.3.1](https://www.github.com/googleapis/python-dataflow-client/compare/v0.3.0...v0.3.1) (2021-11-01)
+## [0.3.1](https://www.github.com/googleapis/python-dataflow-client/compare/v0.3.0...v0.3.1) (2021-11-01)
 
 
 ### Bug Fixes
@@ -66,21 +78,21 @@
 
 * add context manager support in client ([#41](https://www.github.com/googleapis/python-dataflow-client/issues/41)) ([2d8fbd5](https://www.github.com/googleapis/python-dataflow-client/commit/2d8fbd5ff487a5e0aeea8cbf6a4d523861bb197a))
 
-### [0.1.5](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.4...v0.1.5) (2021-09-30)
+## [0.1.5](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.4...v0.1.5) (2021-09-30)
 
 
 ### Bug Fixes
 
 * improper types in pagers generation ([ce0077b](https://www.github.com/googleapis/python-dataflow-client/commit/ce0077b8b99566a6c97dbbd355eb9a0ea75f5ed3))
 
-### [0.1.4](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.3...v0.1.4) (2021-09-24)
+## [0.1.4](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.3...v0.1.4) (2021-09-24)
 
 
 ### Bug Fixes
 
 * add 'dict' annotation type to 'request' ([db5d966](https://www.github.com/googleapis/python-dataflow-client/commit/db5d966111077c2ba286136b9b5aba6a371c8e0d))
 
-### [0.1.3](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.2...v0.1.3) (2021-08-18)
+## [0.1.3](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.2...v0.1.3) (2021-08-18)
 
 
 ### Features
@@ -97,14 +109,14 @@
 
 * release as 0.1.3 ([#19](https://www.github.com/googleapis/python-dataflow-client/issues/19)) ([5fa4e3f](https://www.github.com/googleapis/python-dataflow-client/commit/5fa4e3fe0a17f71c719fe5dc093809bb73600b2d))
 
-### [0.1.2](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.1...v0.1.2) (2021-07-21)
+## [0.1.2](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.1...v0.1.2) (2021-07-21)
 
 
 ### Bug Fixes
 
 * **deps:** pin 'google-{api,cloud}-core', 'google-auth' to allow 2.x versions ([#13](https://www.github.com/googleapis/python-dataflow-client/issues/13)) ([a85e8aa](https://www.github.com/googleapis/python-dataflow-client/commit/a85e8aac12a35a6bf82c2414d8c4018b64d36cc4))
 
-### [0.1.1](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.0...v0.1.1) (2021-06-30)
+## [0.1.1](https://www.github.com/googleapis/python-dataflow-client/compare/v0.1.0...v0.1.1) (2021-06-30)
 
 
 ### Miscellaneous Chores
diff --git a/setup.py b/setup.py
index e637645..b15a060 100644
--- a/setup.py
+++ b/setup.py
@@ -21,7 +21,7 @@
 
 name = "google-cloud-dataflow-client"
 description = "Cloud Dataflow API client library"
-version = "0.5.1"
+version = "0.5.2"
 release_status = "Development Status :: 4 - Beta"
 url = "https://github.com/googleapis/python-dataflow-client"
 dependencies = [
@@ -29,7 +29,8 @@
     # Until this issue is closed
     # https://github.com/googleapis/google-cloud-python/issues/10566
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
-    "proto-plus >= 1.15.0",
+    "proto-plus >= 1.15.0, <2.0.0dev",
+    "protobuf >= 3.19.0, <4.0.0dev",
 ]
 
 package_root = os.path.abspath(os.path.dirname(__file__))
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index be5a64f..786e637 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -6,3 +6,4 @@
 # Then this file should have google-cloud-foo==1.14.0
 google-api-core==1.31.5
 proto-plus==1.15.0
+protobuf==3.19.0
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
index e69de29..786e637 100644
--- a/testing/constraints-3.7.txt
+++ b/testing/constraints-3.7.txt
@@ -0,0 +1,9 @@
+# This constraints file is used to check that lower bounds
+# are correct in setup.py
+# List all library dependencies and extras in this file.
+# Pin the version to the lower bound.
+# e.g., if setup.py has "google-cloud-foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have google-cloud-foo==1.14.0
+google-api-core==1.31.5
+proto-plus==1.15.0
+protobuf==3.19.0
diff --git a/tests/unit/gapic/dataflow_v1beta3/test_flex_templates_service.py b/tests/unit/gapic/dataflow_v1beta3/test_flex_templates_service.py
index 220ac9d..88c0994 100644
--- a/tests/unit/gapic/dataflow_v1beta3/test_flex_templates_service.py
+++ b/tests/unit/gapic/dataflow_v1beta3/test_flex_templates_service.py
@@ -13,9 +13,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import math
 import os
 
+# try/except added for compatibility with python < 3.8
+try:
+    from unittest import mock
+    from unittest.mock import AsyncMock
+except ImportError:
+    import mock
+
+import math
+
 from google.api_core import gapic_v1, grpc_helpers, grpc_helpers_async, path_template
 from google.api_core import client_options
 from google.api_core import exceptions as core_exceptions
@@ -25,7 +33,6 @@
 from google.oauth2 import service_account
 import grpc
 from grpc.experimental import aio
-import mock
 from proto.marshal.rules.dates import DurationRule, TimestampRule
 import pytest
 
diff --git a/tests/unit/gapic/dataflow_v1beta3/test_jobs_v1_beta3.py b/tests/unit/gapic/dataflow_v1beta3/test_jobs_v1_beta3.py
index 1f06d78..788b19f 100644
--- a/tests/unit/gapic/dataflow_v1beta3/test_jobs_v1_beta3.py
+++ b/tests/unit/gapic/dataflow_v1beta3/test_jobs_v1_beta3.py
@@ -13,9 +13,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import math
 import os
 
+# try/except added for compatibility with python < 3.8
+try:
+    from unittest import mock
+    from unittest.mock import AsyncMock
+except ImportError:
+    import mock
+
+import math
+
 from google.api_core import gapic_v1, grpc_helpers, grpc_helpers_async, path_template
 from google.api_core import client_options
 from google.api_core import exceptions as core_exceptions
@@ -29,7 +37,6 @@
 from google.protobuf import timestamp_pb2  # type: ignore
 import grpc
 from grpc.experimental import aio
-import mock
 from proto.marshal.rules.dates import DurationRule, TimestampRule
 import pytest
 
diff --git a/tests/unit/gapic/dataflow_v1beta3/test_messages_v1_beta3.py b/tests/unit/gapic/dataflow_v1beta3/test_messages_v1_beta3.py
index 35c5396..27f1fa6 100644
--- a/tests/unit/gapic/dataflow_v1beta3/test_messages_v1_beta3.py
+++ b/tests/unit/gapic/dataflow_v1beta3/test_messages_v1_beta3.py
@@ -13,9 +13,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import math
 import os
 
+# try/except added for compatibility with python < 3.8
+try:
+    from unittest import mock
+    from unittest.mock import AsyncMock
+except ImportError:
+    import mock
+
+import math
+
 from google.api_core import gapic_v1, grpc_helpers, grpc_helpers_async, path_template
 from google.api_core import client_options
 from google.api_core import exceptions as core_exceptions
@@ -26,7 +34,6 @@
 from google.protobuf import timestamp_pb2  # type: ignore
 import grpc
 from grpc.experimental import aio
-import mock
 from proto.marshal.rules.dates import DurationRule, TimestampRule
 import pytest
 
diff --git a/tests/unit/gapic/dataflow_v1beta3/test_metrics_v1_beta3.py b/tests/unit/gapic/dataflow_v1beta3/test_metrics_v1_beta3.py
index f431d91..74168fe 100644
--- a/tests/unit/gapic/dataflow_v1beta3/test_metrics_v1_beta3.py
+++ b/tests/unit/gapic/dataflow_v1beta3/test_metrics_v1_beta3.py
@@ -13,9 +13,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import math
 import os
 
+# try/except added for compatibility with python < 3.8
+try:
+    from unittest import mock
+    from unittest.mock import AsyncMock
+except ImportError:
+    import mock
+
+import math
+
 from google.api_core import gapic_v1, grpc_helpers, grpc_helpers_async, path_template
 from google.api_core import client_options
 from google.api_core import exceptions as core_exceptions
@@ -26,7 +34,6 @@
 from google.protobuf import timestamp_pb2  # type: ignore
 import grpc
 from grpc.experimental import aio
-import mock
 from proto.marshal.rules.dates import DurationRule, TimestampRule
 import pytest
 
diff --git a/tests/unit/gapic/dataflow_v1beta3/test_snapshots_v1_beta3.py b/tests/unit/gapic/dataflow_v1beta3/test_snapshots_v1_beta3.py
index 11ee5cd..b6213bb 100644
--- a/tests/unit/gapic/dataflow_v1beta3/test_snapshots_v1_beta3.py
+++ b/tests/unit/gapic/dataflow_v1beta3/test_snapshots_v1_beta3.py
@@ -13,9 +13,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import math
 import os
 
+# try/except added for compatibility with python < 3.8
+try:
+    from unittest import mock
+    from unittest.mock import AsyncMock
+except ImportError:
+    import mock
+
+import math
+
 from google.api_core import gapic_v1, grpc_helpers, grpc_helpers_async, path_template
 from google.api_core import client_options
 from google.api_core import exceptions as core_exceptions
@@ -27,7 +35,6 @@
 from google.protobuf import timestamp_pb2  # type: ignore
 import grpc
 from grpc.experimental import aio
-import mock
 from proto.marshal.rules.dates import DurationRule, TimestampRule
 import pytest
 
diff --git a/tests/unit/gapic/dataflow_v1beta3/test_templates_service.py b/tests/unit/gapic/dataflow_v1beta3/test_templates_service.py
index ba480bb..4fada5f 100644
--- a/tests/unit/gapic/dataflow_v1beta3/test_templates_service.py
+++ b/tests/unit/gapic/dataflow_v1beta3/test_templates_service.py
@@ -13,9 +13,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import math
 import os
 
+# try/except added for compatibility with python < 3.8
+try:
+    from unittest import mock
+    from unittest.mock import AsyncMock
+except ImportError:
+    import mock
+
+import math
+
 from google.api_core import gapic_v1, grpc_helpers, grpc_helpers_async, path_template
 from google.api_core import client_options
 from google.api_core import exceptions as core_exceptions
@@ -27,7 +35,6 @@
 from google.rpc import status_pb2  # type: ignore
 import grpc
 from grpc.experimental import aio
-import mock
 from proto.marshal.rules.dates import DurationRule, TimestampRule
 import pytest