The heterochromatin protein 1 (HP1) family of proteins is involved in gene silencing via the formation of heterochromatic structures. They are composed of two related domains: an N‐terminal chromo domain and a C‐terminal shadow chromo domain. Present results suggest that chromo domains may function as protein interaction motifs, bringing together different proteins in multi‐protein complexes and locating them in heterochromatin. We have previously determined the structure of the chromo domain from the mouse HP1β protein, MOD1. We show here that, in contrast to the chromo domain, the shadow chromo domain is a homodimer. The intact HP1β protein is also dimeric, where the interaction is mediated by the shadow chromo domain, with the chromo domains moving independently of each other at the end of flexible linkers. Mapping studies, with fragments of the CAF1 and TIF1β proteins, show that an intact, dimeric, shadow chromo domain structure is required for complex formation.

HP1 family proteins are adaptor molecules, containing two related chromo domains that are required for chromatin packaging and gene silencing. Here we present the structure of the chromo shadow domain from mouse HP1β bound to a peptide containing a consensus PXVXL motif found in many HP1 binding partners. The shadow domain exhibits a novel mode of peptide recognition, where the peptide binds across the dimer interface, sandwiched in a β‐sheet between strands from each monomer. The structure allows us to predict which other shadow domains bind similar PXVXL motif‐containing peptides and provides a framework for predicting the sequence specificity of the others. We show that targeting of HP1β to heterochromatin requires shadow domain interactions with PXVXL‐containing proteins in addition to chromo domain recognition of Lys‐9‐methylated histone H3. Interestingly, it also appears to require the simultaneous recognition of two Lys‐9‐methylated histone H3 molecules. This finding implies a further complexity to the histone code for regulation of chromatin structure and suggests how binding of HP1 family proteins may lead to its condensation.

Membrane proteins pose a huge challenge for structural analysis, but a new study reports the first NMR structure determination of a detergent-solubilized seven-helical transmembrane (7TM) protein, the phototaxis receptor sensory rhodopsin II. This case study may open the doors to similar solution NMR structures for other 7TM proteins. Seven-helix membrane proteins represent a challenge for structural biology. Here we report the first NMR structure determination of a detergent-solubilized seven-helix transmembrane (7TM) protein, the phototaxis receptor sensory rhodopsin II (pSRII) from Natronomonas pharaonis, as a proof of principle. The overall quality of the structure ensemble is good (backbone r.m.s. deviation of 0.48 Å) and agrees well with previously determined X-ray structures. Furthermore, measurements in more native-like small phospholipid bicelles indicate that the protein structure is the same as in detergent micelles, suggesting that environment-specific effects are minimal when using mild detergents. We use our case study as a platform to discuss the feasibility of similar solution NMR studies for other 7TM proteins, including members of the family of G protein–coupled receptors.