The SPASM domain usually occurs as a C-terminal extension to a radical SAM domain (see Pfam model PF04055)-containing protein, although some proteins with free-standing SPASM domains do occur. A radical SAM protein binds a 4Fe-4S cluster, and the SPASM domain binds two additional 4Fe-4S clusters. Radical SAM enzymes with an additional SPASM domain tend to be involved in protein modification. The acronym SPASM derives from the protein-modifying radical SAM enzymes of the Subtilosin, PQQ, Anaerobic Sulfatase, and Mycofactocin systems. Additional radical SAM/SPASM domain proteins participate in the maturation of quinohemoprotein amine dehydrogenase, the natural product Pep1357, SCIFF, sporulation killing factor, thurincin H, thuricin CD, etc. The motif CxxCxxxxxCxxxC is nearly invariant for members of this family, although PqqE has a variant form. An incomplete SPASM domain that binds one 4Fe-4S cluster, instead of two, is called a TWITCH domain. This domain contains regions binding additional 4Fe4S clusters found in various radical SAM proteins C-terminal to the domain described by model PF04055. Radical SAM enzymes with this domain tend to be involved in protein modification, including anaerobic sulfatase maturation proteins, a quinohemoprotein amine dehydrogenase biogenesis protein, the Pep1357-cyclizing radical SAM enzyme, and various bacteriocin biosynthesis proteins. The motif CxxCxxxxxCxxxC is nearly invariant for members of this family, although PqqE has a variant form. We name this domain SPASM for Subtilosin, PQQ, Anaerobic Sulfatase, and Mycofactocin.
- Date:
- 2021-10-28