Gap junctional proteins of animals: the innexin/pannexin superfamily

Abstract

There has been some controversy as to whether vertebrate pannexins are related to invertebrate innexins. Using statistical, topological and conserved sequence motif analyses, we establish that these proteins belong to a single superfamily. We also demonstrate the occurrence of large homologues with C-terminal proline-rich domains that may have arisen by gene fusion events. Phylogenetic analyses reveal the orthologous and paralogous relationships of these homologues to each other. We show that different sets of orthologous paralogues underwent sequence divergence at markedly different rates, suggesting differential pressures through evolutionary time promoting or restricting sequence divergence. We further show that the first 2 TMS-containing halves of these homologues underwent sequence divergence more slowly than the second 2 TMS-containing halves and analyze these differences. These bioinformatic analyses should serve as useful guides for future studies of structure, function and evolutionary aspects of this important superfamily.

Fig. 1

Alignment of almost all of Hsa3 (pannexin 3 of man; top sequence) with almost all of Hme2 (innexin 2 of the leech; bottom sequence). The GAP program (Devereux et al., 1984) was used to generate the alignment which gave a comparison score of 17 S.D. (see Table 2). Numbers at the beginning and end of each line indicate residue numbers in the proteins. *, identity; :, close similarity, ●, more distant similarity as defined by the GAP program. Motifs 1 and 2 which align are boxed and shaded. Motif 3 which does not align in the two sequences is overlined (top sequence) or underlined (bottom sequence).

Fig. 2

Alignment of the second fully conserved pair of cysteyl residues in the pannexins (Hsu3, Pan, top) with the fully conserved pair of cysteyl residues in the innexins (Hmo2, Inn, bottom). The alignment, taken from Figure 1, shows 20% identity and 45% similarity. Vertical lines represent identities while colons represent similarities.

Fig. 3

Phylogenetic tree of representative vertebrate pannexins and invertebrate innexins which cluster separately as labeled. The three pannexin isoforms (pannexins 1, 2 and 3) are so designated. The innexins cluster according to organismal source type. The pannexins and innexins are shaded separately.

Fig. 4

Average hydropathy (dark line; top) and average similarity (thin line, bottom) for the Innexins (A) and the Pannexins (B). The AveHAS program (Zhai and Saier, 2001) was used to derive the plots. The CLUSTAL X-derived multiple alignments (Figs. S2 and S3) presented on our website (http://biology.ucsd.edu/~msaier/supmat/InnPan) were used to derive these plots.