I have a Pubsub topic say top1 in proj1 with a subscription existing already and it its full qualified name say is proj1/subscriptions/sub1

I have a service account in a different project proj2 and this account is called sa2. sa2 was granted the following permissions

1. roles/pubsub.editor
2. roles/pubsub.publisher
3. roles/pubsub.subscriber

I went ahead and created a new subscription on top1 in proj1 using the cli and creds of sa2 and let us call it proj2/subscriptions/sub2.
Everything works fine when I try to use JAVA SDK and consume from proj2/subscriptions/sub2 as top1 in proj1 has this new subscription that I created.

However, I do not in long run want to create a subscription. I want to use an existing subscription in the other project ie proj1/subscriptions/sub1 with sa2. But when I try that with above permissions granted, I am getting a permission denied.

Pasting just a small part of trace

com.google.api.gax.rpc.PermissionDeniedException: 
com.google.api.gax.rpc.PermissionDeniedException: io.grpc.StatusRuntimeException: 
PERMISSION_DENIED: User not authorized to perform this action.