PARcomplex

PARcomplex refers to a conserved set of proteins that establish and maintain polarized states in animal cells by localizing to distinct regions of the cell cortex. The term originates from partitioning-defective mutants identified in Caenorhabditis elegans and describes a modular polarity system that operates across diverse metazoans to separate anterior and posterior cortical domains.

Core components and organization typically include PAR-3, PAR-6, and an atypical protein kinase C (aPKC). In mammals these are represented by PARD3, PARD6A/B, and PRKCI or PRKCZ. PAR-3 acts as a scaffold that recruits PAR-6 and aPKC; the complex is regulated by small GTPases such as Cdc42. The activity of aPKC and its phosphorylation of substrates help restrict the anterior PAR complex to the cortex while posterior PAR proteins, such as PAR-1 and PAR-2, occupy the opposite domain. The two sets of proteins mutually antagonize each other, creating distinct anterior and posterior cortical domains.

Functions of the PAR complex include establishing cell polarity during early embryogenesis, organizing epithelial and neuronal polarity, guiding asymmetric cell division, and coordinating cytoskeletal dynamics and vesicle trafficking. Proper PAR complex function interacts with actin and microtubule networks and is modulated by signaling cascades involving Rho family GTPases.

Evolution and relevance: the PAR system is highly conserved in animals, with variations in the exact protein complements across lineages. Disruption of PAR polarity components is associated with developmental defects and has implications for tissue organization and disease when polarity is compromised.