MempromCC: A new family of membrane-attached coiled coils in prokaryotes and mitochondria

We have recently discovered a hitherto uncharacterized group of proteins consisting of a helical head, one or multiple β-layer necks, a coiled-coil stalk and a trans-membrane anchor (HNSA architecture). We have named it MempromCC, for membrane-attached coiled-coil proteins of prokaryotes and mitochondria. While many known protein families have similar architectures, the β-layer neck found between the head and the coiled coil is a unique and persistent characteristic of this group. Bioinformatic analysis of protein sequences with a predicted HNSA architecture showed that, even though the neck appears to be strikingly similar in most sequences and the helical head is conserved in many organisms, MempromCC proteins display great variation in size, sequence and structure. This is particularly conspicuous in the coiled-coil stalks, which show a range of periodicities, lengths, and numbers of β-layers. This variability and lack of convincing sequence similarity across all MempromCC proteins suggest that the group may also include some analogous proteins. However, a core subgroup of proteins from proteobacteria and mitochondria, defined by conserved head sequence motifs, clearly forms a homologous family. Although these proteins have not been functionally characterized and are mostly annotated as hypothetical, two of them, named MCUR1 (or CCDC90A) and Rat1, have been associated with the assembly of membrane complexes in organelles