Mechanistic Insights into HP1α CSD Oligomerization and the Role of <i>PxVxL</i> Motif-Containing Proteins

HP1 oligomerization is crucial for chromatin compaction, transcriptional regulation, and heterochromatin maintenance. The dynamic transition between HP1 monomeric, dimeric, and higher-order oligomeric states modulates the chromatin structure and phase separation. By controlling the HP1 oligomerization dynamics, chromatin-associated processes involving HP1 can be regulated. In this study, we employed molecular dynamics simulations to investigate HP1α chromo shadow domain (CSD) oligomerization, focusing on how <i>PxVxL</i> motif-containing partner KAP1 and flanking residues (FRs) modulate dimerization at the α-helix and β-sheet interfaces. Our findings reveal that in the monomeric state the C- and N-tails remain frequently in contact with the β-sheet face, restricting its accessibility and favoring dimerization at the α-helix face. Dimerization at the α-helix face creates the <i>PxVxL</i> motif binding cavity and suppresses intramonomeric C-tail-β-sheet interactions. However, intermonomeric C-tail-β-sheet interactions are enhanced, thereby opening the β-sheet face partially. Following the KAP1 binding, the C-tail-β-sheet interactions are suppressed and N-tail flexibility is promoted, making the β-sheet face available for dimerization. Dimerization at the β-sheet face involves interactions between the β-sheet face residues from two CSD units, of which the hydrophobic interactions mediated by L139 and L150 and polar interactions mediated by R115 are critical. KAP1 FRs interact with the β-sheet residues laterally. Overall, our comparative analysis of KAP1-bound and unbound CSD dimers elucidates the molecular mechanism of HP1 oligomerization and highlights how KAP1 binding drives structural rearrangements that modulate HP1's function as an adaptable molecular scaffold in heterochromatin formation.