This family (previously DUF4555) includes the homologs of human sperm-associated microtubule inner protein 4 (SPMIP4, also known as C7orf31) that is found predominantly in testis []. SPMIP4 is a microtubule inner protein (MIP) part of the axonemal dynein-decorated doublet microtubules (DMTs).Members of this family adopt rather extended conformation and contain a single or multiple Mn motifs. Each Mn motif consists of a 5- to 6-residue helix flanked by a Tyr/Phe residue on one end and by a Ser/Thr on the other that binds the shoulder of the ��-tubulin S9/S10 loop []. It was reported that a single Mn motif is sufficient for microtubule binding in vitro []. Members of this family are likely to stabilise the microtubule structure in the sperm flagella [].
