PAK Kinases Target Sortilin and Modulate Its Sorting

Abstract

The multifunctional type 1 receptor sortilin is involved in endocytosis and intracellular transport of ligands. The short intracellular domain of sortilin binds several cytoplasmic adaptor proteins (e.g., the AP-1 complex and GGA1 to -3), most of which target two well-defined motifs: a C-terminal acidic cluster dileucine motif and a YXXΦ motif in the proximal third of the domain. Both motifs contribute to endocytosis as well as Golgi-endosome trafficking of sortilin. The C-terminal acidic cluster harbors a serine residue, which is subject to phosphorylation by casein kinase. Phosphorylation of this serine residue is known to modulate adaptor binding to sortilin. Here, we show that the cytoplasmic domain of sortilin also engages Rac-p21-activated kinases 1 to 3 (PAK1-3) via a binding segment that includes a tyrosine-based motif, also encompassing a serine residue. We further demonstrate that PAK1-3 specifically phosphorylate this serine residue and that this phosphorylation alters the affinity for AP-1 binding and consequently changes the intracellular localization of sortilin as a result of modulated trafficking. Our findings suggest that trafficking of ligands bound to sortilin is in part regulated by group A PAK kinases, which are downstream effectors of Rho GTPases and are known to affect a variety of processes by remodeling the cytoskeleton and by promoting gene transcription and cell survival.

AP-3–dependent targeting of flippase ATP8A1 to lamellar bodies suppresses activation of YAP in alveolar epithelial type 2 cells

“…Furthermore, we suspect that posttranslational modifications are necessary for the AP-3/ATP8A1 interaction. While our studies demonstrate that 1105 ERAQLL is constitutively active like most di-leucine-based sorting signals, the presence of a favorable phosphorylation site in close proximity [PhosphoSitePlus (62); site group ID 4712266] suggests that dynamic phosphorylation may impact AP recognition and/or function (61,63,64).…”

Sorting through the extensive and confusing roles of sortilin in metabolic disease

“…Phosphofurin acidic cluster sorting protein 1 (PACS-1) binds phosphorylated acidic clusters, mediating retrograde Golgi-endosome transport (105), and may play a role in the retrieval of sortilin (69,106). The YXXΦ motif (YSVL) also contains a serine residue, one that can be phosphorylated by Rac-p21-activated kinase (PAKs) 1-3 (107).…”

Mechanisms regulating the sorting of soluble lysosomal proteins

“…The cytosolic tail of sortilin is the site of various forms of post-translational modifications (PTMs) regulating its function. Phosphorylation can occur at serine 793 via the p21-activated kinase family [ 74 ]. This modification appears to modulate the interaction between sortilin and Adaptor Protein 1 (AP-1), as constitutively active phosphomimetic constructs of sortilin were unable to bind AP-1 in a yeast 2-hybrid system [ 74 ].…”

“…Phosphorylation can occur at serine 793 via the p21-activated kinase family [ 74 ]. This modification appears to modulate the interaction between sortilin and Adaptor Protein 1 (AP-1), as constitutively active phosphomimetic constructs of sortilin were unable to bind AP-1 in a yeast 2-hybrid system [ 74 ]. A second phosphorylation site in the cytosolic tail of sortilin has also been identified.…”