Biology subjects

Bhakta, K.

Publications and source records attributed to Bhakta, K..

4 recordsLinked to original sources

bioRxiv2025

A conserved Hsp70 phosphorylation regulates cell cycle progression after DNA damage

Hsp70s are essential molecular chaperones that are increasingly recognized to be regulated by post-translational modifications. Here, we show that phosphorylation of a conserved threonine (T495), previously shown to be exploited by a Legionella pneumophila kinase to inhibit Hsp70, occurs endogenously in human cells in response to DNA damage, particularly when base excision repair is overburdened. This modification is cell cycle dependent, and in yeast, phosphomimetic or phosphonull Hsp70 variants disrupt G1/S progression under normal and DNA-damaging conditions. Biochemically, the phosphomimetic T495E mutation locks Hsp70 in an open-like conformation without blocking substrate engagement. Together, our results reveal a conserved mechanism by which dynamic Hsp70 phosphorylation regulates the G1/S transition, and delays cell cycle progression during DNA damage, highlighting how pathogen-derived insights can uncover fundamental cell biology principles.

cell biology↗

bioRxiv2024

Cellular moonlighting function of Hsp20 directs morphological and pathogenic development in Ustilago maydis

Ustilago maydis Hsp20 is involved in the pathogenicity of the fungus. In this study we have investigated the molecular basis of contribution of Hsp20 to U. maydis pathogenicity. Through biochemical studies we have demonstrated environment-dependent oligomeric plasticity associated with Hsp20. Hsp20 was also found to form higher order oligomers that undergo phase separation in vitro. Within cells Hsp20 was found to form distinct punctate structures that we believe play a pivotal role in its function. These punctate structures were demonstrated to sequester proteins such as actin and septin within it. Absence of Hsp20 was found to significantly affect key cellular processes like endocytosis, budding, cell polarity determination and mating in U. maydis cells. The deletion mutant failed to sporulate and complete pathogenic life cycle. This study presents a comprehensive understanding of the pathogenic development of U. maydis in reference to the moonlighting function of Hsp20 within the cell.

microbiology↗

bioRxiv2024

Functional diversity in the Hsp60 of Sulfolobus acidocaldarius: mosaic of Group I and Group II chaperonin

External stress can disrupt protein homeostasis in organisms, necessitating the involvement of heat shock proteins (Hsps) to restore equilibrium and ensure survival. Unlike other organisms, the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius lacks Hsp100, Hsp90, and Hsp70, possessing only two small heat shock proteins (Hsp14 and Hsp20) and one group II chaperonin, Hsp60. This raises questions about how protein substrates are protected and transferred to Hsp60 for refolding without other chaperones. Our study focused on ATP-dependent Hsp60 in S. acidocaldarius, revealing its formation of oligomeric structures in the presence of ATP. While ATP hydrolysis is not essential for oligomer formation and lid closure, it is crucial for Hsp60s chaperone activity, effectively folding stress-denatured substrate proteins by stabilizing their folded conformations. The mechanism involves hydrophobic recognition of unfolded substrates, encapsulating and releasing them in a more folded state. Negatively charged inner surface of the ring seems to be responsible for driving the folding of the substrate. Importantly, Hsp14 was found to transfer substrate proteins to Hsp60{beta}, orchestrating their refolding into an active state. Beyond protein folding, Hsp60{beta} protects the membrane under stress, contributing to maintaining membrane rigidity. Hsp60 exhibits nested cooperativity in ATPase activity, adapting to ATP concentration changes and interestingly Hsp60{beta} and Hsp60{beta} complex shows a mosaic behaviour during ATP hydrolysis belonging to both group I and group II chaperonin respectively. In conclusion, our study provides insights into the intricate mechanisms employed by Hsp60 in S. acidocaldarius to maintain protein homeostasis. It offers a comprehensive understanding of Hsp60s role in the heat shock response pathway, shedding light on fundamental cellular processes in extremophilic archaea.

biochemistry↗

bioRxiv2022

N-terminal helices and A domain of archaeal FtsY facilitate SRP54 binding and the association with cell membrane

The process of protein translocation is essential to the maintenance of cellular life and has been critically addressed in eukaryotes and bacteria. However, little information is available regarding protein translocation across archaeal membranes. The signal recognition particle (SRP) plays an important role in this process. It binds the signal peptide at the N-terminus of the polypeptide chain and interacts with the cognate SRP receptor (FtsY) located on the target membrane to form a targeting complex (TC). Concomitant GTP hydrolysis by SRP and FtsY delivers the polypeptide to the adjacent protein-conducting channel. The present study aims to characterize the structural domains of FtsY contributing to the targeting complex (TC) formation in Sulfolobus acidocaldarius, a thermo-acidophilic crenarchaeon. The contacting residues between SRP54 and FtsY were mapped along the N1-N3 helices. Interestingly, the previously reported crystal structure did not take the N-terminal A domain into account - a region rich in negatively charged residues. Such observation led us to investigate the contribution of each of the three participating helices (N1-3) in terms of membrane association and functional TC formation. Through biophysical analyses of SRP-FtsY and FtsY-membrane interaction, and biochemical characterization of the reciprocal GTPase activity, this work sought to elucidate the minimal structural motif controlling the archaeal TC assembly.

biochemistry↗