Small heat shock protein which functions as a molecular chaperone probably maintaining denatured proteins in a folding-competent state. Plays a role in stress resistance and actin organization (PubMed:17661394). Through its molecular chaperone activity may regulate numerous biological processes including the phosphorylation and the axonal transport of neurofilament proteins (By similarity). Homooligomer. Homodimer; becomes monomeric upon activation. Heterooligomer; with HSPB6. Associates with alpha- and beta-tubulin (By similarity). Interacts with TGFB1I1 (PubMed:11546764). Interacts with CRYAB (By similarity). Interacts with HSPB8 (PubMed:11342557). Interacts with HSPBAP1 (PubMed:10751411). Cytoplasmic in interphase cells. Colocalizes with mitotic spindles in mitotic cells. Translocates to the nucleus during heat shock and resides in sub-nuclear structures known as SC35 speckles or nuclear splicing speckles. Additional isoforms seem to exist. Phosphorylated upon exposure to protein kinase C activators and heat shock (By similarity). Phosphorylation by MAPKAPK2 and MAPKAPK3 in response to stress dissociates HSPB1 from large small heat-shock protein (sHsps) oligomers and impairs its chaperone activity and ability to protect against oxidative stress effectively. Phosphorylation by MAPKAPK5 in response to PKA stimulation induces F-actin rearrangement (PubMed:1332886, PubMed:1860870, PubMed:21575178, PubMed:8093612). Mice are viable and fertile with no apparent morphological abnormalities in tissues under physiological conditions. Belongs to the small heat shock protein (HSP20) family.