9.B.306. The Putative Calcium Binding Protein with Similarity to the C-termini of Anoctamins (CBP) Family
This family usually consist of short protein of about 140 - 160 aas with 5 TMSs. The last 4 TMSs of these proteins resemble the C-terminal 4 TMSs in the Anoctamins of subfamily 1.A.17.5. The last 4 TMSs of both (sub)families occur in a 2 + 2 TMS arrangement, and in the annoctamins, they comprise a Ca2+ binding site that controls channel activity. The channel immediately precedes the calcium binding domain. Therefore, the small proteins of the CBP family may be calcium binding proteins.
One of these proteins, 9.B.306.1.1, gave a BLAST score of e-6 with 1.A.17.5.6 (25% identity) in the Anoctamin superfamily, again with the last 4 TMSs of both proteins aligning. Using the complexity filter and/or the composition filter did not decrease the BLAST score as it remained 10-6. This supports the conclusion that the small proteins of the 9.B.306 family are calcium binding proteins.
One small protein (9.B.306.1.7) of 65 aas, has 2 TMSs that correspond to the last two TMSs in the other family members. It is not known if the former protein is a fragment or a full length protein with just half as many TMSs as usual. Additionally, the 4 or 5 TMSs of the CBP family proteins may be distantly related to rhomboid proteases (i.e., TC# 9.B.104.1.12) and certain membrane anchored dehydrogenases (i.e., TC# 5.B.2.3.3).