Ubiquitin-protein ligase E3B
ubiquitin protein ligase activity
UBE3B Is a Calmodulin-regulated, Mitochondrion-associated E3 Ubiquitin Ligase.
Recent genome-wide studies found that patients with hypotonia, developmental delay, intellectual disability, congenital anomalies, characteristic facial dysmorphic features, and low cholesterol levels suffer from <u>K</u>aufman <u>o</u>culocerebrofacial <u>s</u>yndrome (KOS, also reported as blepharophimosis-ptosis-intellectual disability syndrome). The primary cause of KOS is autosomal recessive mutations in the gene <i>UBE3B</i> However, to date, there are no studies that have determined the cellular or enzymatic function of UBE3B. Here, we report that UBE3B is a mitochondrion-associated protein with <u>h</u>omologous to the <u>E</u>6-AP <u>C</u><u>t</u>erminus (HECT) E3 ubiquitin ligase activity. Mutating the catalytic cysteine (C1036A) or deleting the entire HECT domain (amino acids 758-1068) results in loss of UBE3B's ubiquitylation activity. Knockdown of UBE3B in human cells induces changes in mitochondrial morphology and physiology, a decrease in mitochondrial volume, and a severe suppression of cellular proliferation. We also discovered that UBE3B interacts with calmodulin via its N-terminal isoleucine-glutamine (IQ) motif. Deletion of the IQ motif (amino acids 29-58) results in loss of calmodulin binding and a significant increase in the <i>in vitro</i> ubiquitylation activity of UBE3B. In addition, we found that changes in calcium levels <i>in vitro</i> disrupt the calmodulin-UBE3B interaction. These studies demonstrate that UBE3B is an E3 ubiquitin ligase and reveal that the enzyme is regulated by calmodulin. Furthermore, the modulation of UBE3B via calmodulin and calcium implicates a role for calcium signaling in mitochondrial protein ubiquitylation, protein turnover, and disease.