<i>Sorghum bicolor</i> membrane steroid binding protein 1 can bind heme and remodel ER membranes

Plant membrane steroid binding proteins (MSBPs) belong to the membrane-associated progesterone receptors (MAPRs) present in all eukaryotic kingdoms. Plant MSBP proteins have been shown to regulate the function of cytochrome P450 enzymes, bind different steroidal compounds and confer salt tolerance. However, the exact molecular function of plant MSBPs remains elusive. Here we perform phylogenetic analysis of the six MAPR genes encoded in the Sorghum bicolor genome. Of these, four group into a distinct MSBP clade characterized by being N-terminally membrane anchored followed by a cytochrome b5 domain and an extended disordered C-terminal. Biophysical of SbMSBP1 demonstrates that this protein can bind heme, which leads to dimerization potentially through a heme-heme stacking mechanism. We further show using untargeted proteomics that MSBPs are upregulated in both root and shoot tissue upon exposure to salt stress. Based on weighted gene co-expression network analysis (WGCNA) we find that SbMSBP1 abundance clusters with ER remodeling and vesicle transport proteins. We further show that overexpression of SbMSBP1 in Sorghum bicolor protoplasts and tobacco results in formation of structures consistent with organized smooth endoplasmic reticulum (OSER). Our data indicates that SbMSBP1 functions to remodel ER membranes, which may be directly linked to a functional role in stress resilience towards both biotic and abiotic stresses and furthermore could serve as a useful tool for metabolic engineering of ER-scaffolded biosynthetic pathways.