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. Through its molecular chaperone activity may regulate numerous biological processes including the phosphorylation and the axonal transport of neurofilament proteins. 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 (By similarity). 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. Expressed in a variety of tissues. High levels in lung, adrenal, xiphoid, adipose tissue, heart and striated and smooth muscle, lower levels in the CNS. Adult levels are much higher in the slow-twitch soleus muscle than in the fast-twitch rectus femoris and extensor digitorum muscles. Expressed in the soleus and rectus femoris muscles at prenatal day 2, increasing to a maximum at postnatal day 3. Expression in the rectus femoris muscle then decreases reaching adult levels within a few weeks. Phosphorylated upon exposure to protein kinase C activators and heat shock. 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. Belongs to the small heat shock protein (HSP20) family.