9.B.323. The Putative ABC3 Auxiliary Protein (ABC3-AP) Family
The proteins of this family usually have two TMSs, one N-terminal, and one C-terminal. Many but by no means all of them cluster with complete ABC3-type transport systems, most in archaea, but also many in firmicutes and actinobacteria (see several ABC3 system listed under TC# 3.A.1.122). They have a wide range of sizes, from about 300 to about 1000 aas. They are more sequence divergent than the membrane proteins of the ABC3 systems with which they possibly function. The genes for the putative auxiliary proteins are often in an operon together with the membrane exporter and the ATPase, with the auxililary protein encoded by the middle gene. This is the COG1361 family. Some of these proteins are annotated as 'S-layer proteins' or 'ABC transporter' or 'exo-alpha-silalidase' in the NCBI database.