The transmembrane protein 147 (TMEM147) is a novel component of the nicalin-nomo protein complex

Nicastrin and its relative Nicalin (Nicastrin-like protein) are both members of larger protein complexes, namely [gamma]-secretase and the Nicalin-NOMO (Nodal modulator) complex. The [gamma]-secretase complex, which contains Presenilin, APH-1 and PEN-2 in addition to Nicastrin, catalyzes the proteolytic cleavage of the transmembrane domain of various proteins including the [beta]-Amyloid Precursor Protein (APP) and Notch. Nicalin and its binding partner NOMO form a complex which was shown to modulate Nodal signaling in developing zebrafish embryos. Since its experimentally determined native size (200-220 kDa) could not be satisfyingly explained by the molecular weights of Nicalin (60 kDa) and NOMO (130 kDa), we searched in affinity-purified complex preparations for additional components in the low-molecular-weight range. A ~22-kDa protein was isolated and identified by mass spectrometry as transmembrane protein 147 (TMEM147), a novel, highly conserved membrane protein with a putative topology similar to APH-1. Like Nicalin and NOMO it localizes to the endoplasmic reticulum and is expressed during early zebrafish development. Overexpression and knockdown experiments in cultured cells demonstrate a close relationship between the three proteins and suggest that they are components of the same complex. We present evidence that, similar to [gamma]-secretase, its assembly is hierarchical starting with the formation of a Nicalin-NOMO intermediate. Nicalin appears to represent the limiting factor regulating the assembly rate by stabilizing the other two components. We conclude that TMEM147 is a novel core component of the Nicalin-NOMO complex further emphasizing its similarity with [gamma]-secretase.