In biochemistry, a protein trimer is a macromolecular complex formed by three, usually non-covalently bound, macromolecules like proteins or nucleic acids. A homo-trimer would be formed by three identical molecules. A hetero-trimer would be formed by three different macromolecules. Type II Collagen is an example of homo-trimeric protein.
Porins usually arrange themselves in membranes as trimers.