9.B.159 The Inclusion Membrane Protein (IncA) Family
The IncA family includes several proteins that are secreted via the Type III Secretion System (TC# 3.A.6) in chlamydial species. No member of this family has been identified outside of the Chlamydiae. The proteins enterred into TCDB as IncA proteins usually have a 2 or 4 TMS topology but are in general not demonstrably homologous using TC BLAST. They are very divergent in sequence, but they all belong to the IncA family of Pfam. They may be related to mitochodrial Ca2+ uniporters and prokaryotic Mg2+ channels (TC# 1.A.77), and consequently, these IncA homologues may function as divalent cation channels (Lee et al. 2014).
Inc proteins may assemble in the membrane of the inclusion (which separates the bacteria from the host cell cytoplasm) to form specific multi-molecular complexes (Gauliard et al. 2015). IncA plays a critical role in both protecting the inclusion from lysosomal fusion and inducing the homotypic fusion of inclusions. Within IncA are two regions homologous to eukaryotic SNARE (soluble N-ethylmaleimide-sensitive factor attachment receptor) domains referred to as SNARE-like domain 1 (SLD1) and SNARE-like domain 2 (SLD2). The functional core of IncA retains the ability to both inhibit SNARE-mediated fusion and promote the homotypic fusion of Chlamydia inclusions.This core region is composed almost entirely of α-helices and assembles into stable homodimers in solution (Ronzone et al. 2014).