Small heat shock protein Hsp20 contributes to pathogenic development of <i>Ustilago maydis</i> by regulating multiple cellular pathways through its interaction with cytoskeletal proteins

Ustilago maydis causes smut disease in maize. In this study, we investigated the molecular basis of the contribution of small heat shock protein Hsp20 to U. maydis pathogenicity. Through biochemical studies, we demonstrate environment-dependent oligomeric plasticity and phase separation properties associated with purified Hsp20. Using genetic deletion mutants of hsp20, we evaluated the involvement of the protein in major cellular processes leading to the pathogenic development of the fungus. Hsp20 was found to form higher order oligomers in vitro that assemble into liquid condensates. Within cells, Hsp20 formed 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 them. The absence of Hsp20 was found to significantly affect key cellular processes, such as endocytosis, budding, cell polarity determination and mating, in U. maydis cells. The deletion mutant failed to sporulate and complete the pathogenic life cycle. Further studies are needed to obtain mechanistic insights into the regulation of these cellular processes by Hsp20.