1PK1: Hetero SAM domain structure of Ph and Scm

The polycomb group proteins are required for the stable maintenance of gene repression patterns established during development. They function as part of large multiprotein complexes created via a multitude of protein-protein interaction domains. Here we examine the interaction between the SAM domains of the polycomb group proteins polyhomeotic (Ph) and Sex-comb-on-midleg (Scm). Previously we showed that Ph-SAM polymerizes as a helical structure. We find that Scm-SAM also polymerizes, and a crystal structure reveals an architecture similar to the Ph-SAM polymer. These results suggest that Ph-SAM and Scm-SAM form a copolymer. Binding affinity measurements between Scm-SAM and Ph-SAM subunits in different orientations indicate a preference for the formation of a single junction copolymer. To provide a model of the copolymer, we determined the structure of the Ph-SAM/Scm-SAM junction. Similar binding modes are observed in both homo- and heterocomplex formation with minimal change in helix axis direction at the polymer joint. The copolymer model suggests that polymeric Scm complexes could extend beyond the local domains of polymeric Ph complexes on chromatin, possibly playing a role in long range repression.
PDB ID: 1PK1Download
MMDB ID: 158841
PDB Deposition Date: 2003/6/4
Updated in MMDB: 2018/02
Experimental Method:
x-ray diffraction
Resolution: 1.8  Å
Source Organism:
Similar Structures:
Biological Unit for 1PK1: dimeric; determined by author
Molecular Components in 1PK1
Label Count Molecule
Proteins (2 molecules)
Polyhomeotic-proximal Chromatin Protein(Gene symbol: ph-p)
Molecule annotation
SEX Comb on Midleg Cg9495-pa(Gene symbol: Scm)
Molecule annotation
* Click molecule labels to explore molecular sequence information.

Citing MMDB