PARproteiner

PARproteiner, or partitioning-defective proteins, are a conserved set of regulators that control cell polarity in animal cells. They were first identified in Caenorhabditis elegans, where they coordinate the asymmetric distribution of cytoplasmic determinants during early embryogenesis and influence the orientation of the mitotic spindle.

In C. elegans, the PAR network is organized into two cortical complexes. The anterior complex comprises PAR-3, PAR-6 and PKC-3 (a form of atypical protein kinase C), while the posterior complex consists of PAR-1 and PAR-2. PAR-5, a 14-3-3–like protein, associates with PAR-1, and PAR-4 (a kinase homologous to LKB1) participates in regulating polarity. The anterior and posterior complexes mutually exclude each other from the cortex, creating a defined boundary that governs the localization of factors that determine cell fate, such as MEX-5/6, and influences spindle positioning and asymmetric cell division.

The PAR system is not limited to a single organism or context. Similar polarity mechanisms operate in other animals, with homologous proteins participating in epithelial polarity, neuronal development, and oocyte maturation. In many species, the PAR network interacts with cytoskeletal regulators and small GTPases (for example, CDC-42) to coordinate cytoskeletal organization and cortical flows.

Significance and research use: PARproteins serve as a canonical model for understanding how cells establish polarity and orient division. Their roles are broadly conserved across metazoans, and dysregulation of PAR pathways can disrupt development and tissue organization, making them a focus of studies in cell biology, development, and disease.