Into the fold: advances in understanding aPKC membrane dynamics

Abstract

Atypical protein kinase Cs (aPKCs) are part of the PKC family of protein kinases and are atypical because they don't respond to the canonical PKC activators diacylglycerol (DAG) and Ca2+. They are central to the organization of polarized cells and are deregulated in several cancers. aPKC recruitment to the plasma membrane compartment is crucial to their encounter with substrates associated with polarizing functions. However, in contrast with other PKCs, the mechanism by which atypical PKCs are recruited there has remained elusive until recently. Here, we bring aPKC into the fold, summarizing recent reports on the direct recruitment of aPKC to membranes, providing insight into seemingly discrepant findings and integrating them with existing literature.

Keywords: apkc; atypical pkc; cell polarity; phospholipids; protein kinase c.

Figure 1.. The aPKC N-terminal domains are predicted to be organized into a single functional module, the regulatory module (RM), with membrane binding determinants oriented in a single continuous surface.

(A) Representation of the primary structure of aPKC (human aPKCι residue numbering) with protein domains and the three most abundantly detected phosphorylation sites indicated (cf. PhosphoSitePlus database). RM, regulatory module; PB1, Phox and Bem1 domain; PSS, Pseudo-substrate sequence; BSL, beta-strand linker; KD, kinase domain; T412, activation loop residue; T564, turn motif residue. (B) AlphaFold2 colab prediction of the aPKC^RM, with the PB1 domain (cyan), PSS (purple) linked via a Beta-Strand Linker (orange) to the C1 domain (blue). Figure generated in PyMol. (C) Schematic representation of B. (D) Surface representation of the RM with the predicted lipid-binding residues within the RM color coded in yellow. Residues involved in membrane-association as identified in [11] are labeled. Arg150/151 are part of a NLS as identified in [21] and denoted with a red asterisk. Figure generated in ChimeraX. (E) Indication of the Tyr-136 residue location, which in the unphosphorylated state is part of the BSL. Figure panels B–E are adapted from [11].

Figure 2.. Lipid and protein factors that mediate membrane binding and release.

(A) Representation of the lipid binding properties and dependencies on membrane binding regions within the aPKC^RM. The PSS predominantly binds PIs, while the C1 binds PtdSer, PtdGro and other anionic lipids. In neuronal tissues the abundance of the latter likely provides the affinity for the RM to bind via the C1 domain, whereas in other epithelia a combined PI-dependency is likely. (B) Model detailing the conformational states of cytosolic and membrane bound forms of aPKC. Cytosolic and monomeric aPKC exists in an autoinhibited conformation with the PSS and C1 domain engaging the kinase domain. Upon binding of the indicated protein and lipid factors, this inhibitory module becomes exposed and undergoes a conformational change to establish a more compact RM, in which an interdomain BSL (orange) is formed to couple the PB1 and C1 domains; this organizes a rigid membrane binding platform containing residues from both the PSS and C1 domain. Phosphorylation at Tyr-136 in aPKCι driven by Src (and potentially other kinases) results in a loss of the BSL and the integrated membrane platform and consequently membrane affinity. The resulting cytoplasmic kinase is in a more open conformation and potentially subject to pY binding protein modules that may support directed activity. For more details see text.