SAM (sterile alpha motif) domain of Polycomb group is a protein-protein interaction domain. The Polycomb group includes transcriptional repressors which are involved in the regulation of some key regulatory genes during development in many organisms. They are best known for silencing Hox (Homeobox) genes. Polycomb proteins work together in large multimeric and chromatin-associated complexes. They organize chromatin of the target genes and maintain repressed states during many cell divisions. Polycomb proteins are classified based on their common function, but not on conserved domains and/or motifs; however many Polycomb proteins (members of PRC1 class complex) contain SAM domains which are more similar to each other inside of the Polycomb group than to SAM domains outside of it. Most information about structure and function of Polycomb SAM domains comes from studies of Ph (Polyhomeotic) and Scm (Sex comb on midleg) proteins. Polycomb SAM domains usually can be found at the C-terminus of the proteins. Some members of this group contain, in addition to the SAM domain, MTB repeats, Zn finger, and/or DUF3588 domains. Polycomb SAM domains can form homo- and/or heterooligomers through ML and EH surfaces. SAM/SAM oligomers apparently play a role in transcriptional repression through polymerization along the chromosome. Polycomb proteins are known to be highly expressed in some cells years before their cancer pathology; thus they are attractive markers for early cancer therapy.
Feature 1:oligomer interface ML [polypeptide binding site]
Evidence:
Structure:1PK1_A; Drosophila, SAM_Ph/SAM_Scm complex, contacts at 4A.
Comment:The mid-loop (ML) surface of the SAM domain of Ph protein interacts with the end-helix (EH) surface of the SAM domain of Scm protein, forming heterodimers.
Structure:1PK3, Drosophila, SAM domain homooligomer of Scm protein, contacts at 4A.
Comment:The SAM domains of Scm and Ph proteins can form head-to-tail homooligomers via interactions between the ML and EH surfaces.
Jiyao W et al.(2023). "The conserved domain database in 2023.",