SAM (sterile alpha motif) domain of Scm (Sex comb on midleg) subfamily of Polycomb group is a protein-protein interaction domain. Proteins of this subfamily are transcriptional repressors associated with PRC1 complex. This group includes invertebrate Scm protein and chordate Scm homolog 1 and Scm-like 1, 2, 3 proteins. Most have a SAM domain, two MBT repeats, and a DUF3588 domain, except Scm-like 4 proteins which do not have MBT repeats. Originally the Scm protein was described in Drosophila as a regulator required for proper spatial expression of homeotic genes. It plays a major role during early embryogenesis. SAM domains of Scm proteins can interact with each other, forming homooligomers, as well as with SAM domains of other proteins, in particular with SAM domains of Ph (polyhomeotic) proteins, forming heterooligomers. Homooligomers are similar to the ones formed by SAM Pointed domains of the TEL proteins. Such SAM/SAM oligomers apparently play a role in transcriptional repression through polymerization along the chromosome. Mammalian Scmh1 protein is known be indispensible member of PRC1 complex; it plays a regulatory role for the complex during meiotic prophase of male sperm cells, and is particularly involved in regulation of chromatin modification at the XY chromatin domain of the pachytene spermatocytes.
Jiyao W et al.(2023). "The conserved domain database in 2023.",