PAK Kinases Target Sortilin and Modulate Its Sorting.

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.

Structural basis for the binding of naproxen to human serum albumin in the presence of fatty acids and the GA module.

The previously determined crystal structure of the bacterial albumin-binding GA module in complex with human serum albumin (HSA) suggested the possibility of utilizing the complex in the study of ligand binding to HSA. As a continuation of these studies, the crystal structure of the HSA-GA complex with the drug molecule naproxen and the fatty acid decanoate bound to HSA has been determined to a resolution of 2.5 A. In terms of drug binding, the structure suggests that the binding of decanoate to the albumin molecule may play a role in making the haemin site in subdomain IB of the albumin molecule available for the binding of naproxen. In addition, structure comparisons with solved structures of HSA and of the HSA-GA complex show that the GA module is capable of binding to different conformations of HSA. The HSA-GA complex therefore emerges as a possible platform for the crystallographic study of specific HSA-drug interactions and of the influence exerted by the presence of fatty acids.

Crystal structure of a bacterial albumin-binding domain at 1.4 A resolution.

The albumin-binding domain, or GA module, of the peptostreptococcal albumin-binding protein expressed in pathogenic strains of Finegoldia magna is believed to be responsible for the virulence and increased growth rate of these strains. Here we present the 1.4A crystal structure of this domain, and compare it with the crystal structure of the GA-albumin complex. An analysis of protein-protein interactions in the two crystals, and the presence of multimeric GA species in solution, indicate the GA module is "sticky", and is capable of forming contacts with a range of protein surfaces. This might lead to interactions with different host proteins.