bSarcolemmal membrane-associated protein (SLMAP) is a tail-anchored protein involved in fundamental cellular processes, such as myoblast fusion, cell cycle progression, and chromosomal inheritance. Further, SLMAP misexpression is associated with endothelial dysfunctions in diabetes and cancer. SLMAP is part of the conserved striatin-interacting phosphatase and kinase (STRIPAK) complex required for specific signaling pathways in yeasts, filamentous fungi, insects, and mammals. In filamentous fungi, STRIPAK was initially discovered in Sordaria macrospora, a model system for fungal differentiation. Here, we functionally characterize the STRIPAK subunit PRO45, a homolog of human SLMAP. We show that PRO45 is required for sexual propagation and cell-to-cell fusion and that its forkhead-associated (FHA) domain is essential for these processes. Protein-protein interaction studies revealed that PRO45 binds to STRIPAK subunits PRO11 and SmMOB3, which are also required for sexual propagation. Superresolution structured-illumination microscopy (SIM) further established that PRO45 localizes to the nuclear envelope, endoplasmic reticulum, and mitochondria. SIM also showed that localization to the nuclear envelope requires STRIPAK subunits PRO11 and PRO22, whereas for mitochondria it does not. Taken together, our study provides important insights into fundamental roles of the fungal SLMAP homolog PRO45 and suggests STRIPAK-related and STRIPAK-unrelated functions.
Membrane recruitment of protein complexes, cell signaling modules, and enzymes is a critical step for many cellular functions. The family of tail-anchored proteins is recognized for anchoring proteins and vesicles to specific membranes, such as the endoplasmic reticulum (ER) and the outer mitochondrial membrane (1), and tail-anchored proteins are characterized by a C-terminal single transmembrane domain, which is posttranslationally inserted into membranes (2, 3).Sarcolemmal membrane-associated protein (SLMAP) is a tailanchored protein first identified in myocardiac cells (4). In mammals, this protein is known to be involved in myoblast fusion during embryonic development, excitation-contraction coupling in cardiac myocytes, and cell cycle progression (5-8). Furthermore, SLMAP was identified to be a disease gene for Brugada syndrome, a cardiac channelopathy (9). The functional diversity of SLMAP is dependent on alternative splicing, leading to at least four different isoforms of the protein (4, 6, 7, 10). Importantly, gene expression analyses have implicated SLMAP misexpression with endothelial dysfunctions in diabetes, chromosomal aberrations, and cancer (11-14), and currently, SLMAP is the target of lectin-based treatment of drug-resistant cancer cells (15).SLMAP is conserved from yeasts to humans, and characterized fungal SLMAP homologs include Neurospora crassa HAM-4 (hyphal anastomosis 4), Saccharomyces cerevisiae Far9p (factor arrest 9p) and Far10p, as well as Schizosaccharomyces pombe Csc1p (component of SIP complex 1p) (16-18). HAM-4 is essential for vegetativ...