There are viruses that encode for "miniproteins" which are short transmembrane proteins that play crucial roles in the replication of said viruses. Many of these had their open reading frames overlooked initially, as they are mostly less than fifty amino acids in length (and are not homologous in cellular proteins).
Some of the miniproteins form ion channels via oligomerization in membranes; others modulate the activity of cellular transmembrane proteins by binding to them, as well as other miniproteins whose functions are unknown. It has been found that these short transmembrane proteins are very effective at providing a wider range of mechanisms that control cellular activities, and goes to show that there are probably proteins such as these expressed in cells that have yet to be found.
Read more at http://www.ncbi.nlm.nih.gov/pubmed/24742054