Hsp90β is a major chaperone involved in numerous cellular processes. Hundreds of client proteins depend on Hsp90β for proper folding and/or activity. Regulation of Hsp90β is critical to coordinate its tasks and is mediated by several post-translational modifications. Here, we focus on two phosphorylation sites located in the charged linker region of human Hsp90β, Ser226 and Ser255, which have been frequently reported but whose function remains unclear. Targeted measurements by mass spectrometry indicated that intracellular Hsp90β is highly phosphorylated on both sites (>90%). The level of phosphorylation was unaffected by various stresses (e.g., heat shock, inhibition with drugs) that impact Hsp90β activity. Mutating the two serines to alanines increased the amount of proteins interacting with Hsp90β globally and increased the sensitivity to tryptic cleavage in the C-terminal domain. Further investigation revealed that phosphorylation on Ser255 and to a lesser extent on Ser226 is decreased in the conditioned medium of cultured K562 cells, and that a non-phosphorylatable double alanine mutant was secreted more efficiently than the wild type. Overall, our results show that phosphorylation events in the charged linker regulate both the interactions of Hsp90β and its secretion, through changes in the conformation of the chaperone.

In this study, we investigated for the first time the influence of 2-aminoethyl-piperazine-1-carboxylic acid (APCA) and amino-hexanedioic-1-acid (AHDA) on tumor uptake and elimination kinetics of [64 Cu]-radiolabeled gastrin releasing peptide receptors (GRPR) antagonists. Three GRPR antagonists containing the RM26 sequence were synthesized and conjugated with NOTA via different linkers (LK): polyethylene glycol (PEG-neutral), APCA (dicationic) or AHDA (dianionic). The NOTA-LK-RM26 peptides were radiolabeled with 64 Cu to assess their pharmacokinetic and positron emission tomography (PET) imaging properties using PC3 tumor-bearing athymic nude mice. The inhibition constants (Ki ) of the 3 nat Cu/NOTA-LK-RM26 peptides bearing PEG, dicationic and dianionic linkers were 0.98 ± 0.48 nM, 0.95 ± 0.21 nM, and 17.97 ± 2.79 nM, respectively. The [64 Cu] NOTA-LK-RM26 conjugates were prepared with labeling yields superior to 95% and specific activities of 67 to 77 TBq/mmol. The 3 radiopeptides were stable in vivo and showed GRPR-specific uptake in pancreas with a very fast washout of this tissue observed for [64 Cu]-NOTA-AHDA-RM26 peptide. Results from imaging studies displayed specific PC3 tumor uptake for both [64 Cu]-NOTA-APCA- and AHDA-RM26, similar kidney elimination and fast liver washout. Considering their adequate imaging characteristics, [64 Cu]-NOTA-LK-RM26 bearing APCA- and AHDA-linkers are promising candidates for GRPR-targeted PET imaging prostate cancer.

Phage-encoded cell wall peptidoglycan hydrolyzing enzymes, called endolysins, are essential for efficient release of virions from bacteria, and show species-specific killing of the host. We have demonstrated previously that the interaction between N-terminal catalytic and C-terminal cell wall binding domains of mycobacteriophage D29 endolysin makes the enzyme inactive in Escherichiacoli. Here, we demonstrate that such interaction occurs intramolecularly and is facilitated by a charged linker that connects the two domains. We also show that linker composition is crucial for the inactivation of PG hydrolase in E. coli. Such knowledge will immensely help in bioengineering of endolysins with narrow or broad spectrum antimicrobial activity.