Abstract Description

HSP70/HSP90 organizing protein (HOP) is an adapter protein that modulates heat shock protein 70 (HSP70) and HSP90 chaperone activity. HOP is upregulated in various cancer cells, and its role in cellular signaling cascades, including in the signal transducer and activator of transcription 3 (STAT3) pathway, has been recorded by several studies. Aberrant activation of STAT3 by cytokines or growth factors has been implicated in various diseases, including autoimmune disorders, cancer, and inflammatory conditions. The YXXQ motif (where X is any amino acid) is a conserved peptide sequence present in multiple proteins and receptors, facilitating their interaction with the SH2 domain of STAT3. Although preliminary studies have been conducted to examine the role of HOP in the STAT3 signaling pathway. There are no studies on HOP1a, a novel isoform of canonical HOP, and its interaction with STAT3. Herein, we analyzed the potential interaction of a putative YDWQ motif in HOP1a with STAT3 by performing site-directed mutagenesis on key residues (Y26 and Q29), immunoprecipitation, and western blot analysis. Our findings demonstrate that HOP1a interacts with the SH2 domain of STAT3 through the YDWQ motif. Mutation of side chains (R609 and K591) in the SH2 domain of STAT3, known to interact with the phosphotyrosine in the YXXQ motif, resulted in the loss of HOP1a binding to the SH2 domain of STAT3. Y26 phosphorylation and the Q29 residue in the YDWQ motif are important in maintaining a stable interaction of STAT3 with HOP1a. Our study demonstrates that HOP1a binds to the SH2 domain of STAT3 through a pYXXQ motif-mediated mechanism; however, the results suggest that this might not be the sole mode of interaction.