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9.B.15 The 4 TMS YbhQ (YbhQ) Family

YbhQ of E. coli (TC# 9.B.15.1.1) is a 4 TMS protein with N- and C-termini on the inside (Daley et al., 2005). Some annotations suggest YbhQ is a heme/copper-type cytochrome/quinol oxidase subunit, but this could not be substantiated. Homologues are only found in E. coli and other closely related γ-proteobacteria. The 4 TMSs probably arose by duplication of a 2 TMS-encoding genetic element, but this suggestion needs to be confirmed. 

A second family (TC# 9.B.15.2) with 4 TMSs and similar sizes is the DUF1772 family with many members from Actinobacteria.  It is not established that these two subfamilies of 4 TMS proteins are related.

References associated with 9.B.15 family:

Daley, D.O., M. Rapp, E. Granseth, K. Melen, D. Drew, and G. von Heijne. (2005). Global topology analysis of the Escherichia coli inner membrane proteome. Science 308: 1321-1323. 15919996