Regulation of the CUL3 Ubiquitin Ligase by a Calcium-Dependent Co-adaptor

Abstract

The ubiquitin ligase CUL3 is an essential regulator of neural crest specification whose aberrant activation has been linked to autism, schizophrenia, and hypertension. CUL3 exerts its roles by pairing with ∼90 distinct substrate adaptors, yet how the different CUL3-complexes are activated is poorly understood. Here, we show that CUL3 and its adaptor KLHL12 require two calcium-binding proteins, PEF1 and ALG2, for recognition of their substrate SEC31. PEF1 and ALG2 form a target-specific co-adaptor that translates a transient rise in cytosolic calcium levels into more persistent SEC31 ubiquitylation, which in turn triggers formation of large COPII coats and promotes collagen secretion. As calcium also instructs chondrocyte differentiation and collagen synthesis, calcium-dependent control of CUL3 integrates collagen secretion into broader programs of craniofacial bone formation. Our work, therefore, identifies both calcium and CUL3 co-adaptors as important regulators of ubiquitylation events that control human development.

COPII-dependent ER export in animal cells: adaptation and control for diverse cargo

“…As with previous work, we noted in these data the small number of Sec31A-positive puncta evident in many of the images (Gorur et al 2017; Jin et al 2012; McGourty et al 2016). The motile structures observed (Gorur et al 2017) are not shown to undergo long-range transport.…”

“…As with the earlier work, here a small number of large structures were seen in cells overexpressing KLHL12 that labelled for Sec31A, PEF1 and ALG-2 (McGourty et al 2016). This work also showed for the first time that these larger structures can be labelled with antibodies against procollagen and that depletion of either ALG-2 or PEF1 inhibited secretion of procollagen in HT1080 cells overexpressing type I procollagen (McGourty et al 2016). Then, in 2017, the Schekman group reported large, procollagen- and Sec31A-positive structures in KI6 cells (Gorur et al 2017).…”

“…Several publications (Gorur et al 2017; Jin et al 2012; McGourty et al 2016) have defined large COPII positive structures which can be induced by KLHL12 overexpression and these occasionally co-label for procollagen. Some of the evidence that these are bona fide long-range carriers of procollagen does remain equivocal.…”

“…Subsequent work extended the mechanism of KLHL12-mediated ubiquitylation showing that PEF1 and ALG-2 are both subunits of the CUL3-KLHL12 ubiquitin ligase that modifies Sec31A (McGourty et al 2016). ALG-2 has long been known to be a calcium-regulated modulator of COPII (Shibata et al 2007; Yamasaki et al 2006) and this new study introduced the concept of calcium-dependent regulation of COPII ubiquitylation.…”

“…Phosphorylation, however, plays a much wider role in the regulation of COPII as we will discuss later. More recent work has shown that, as with ubiquitylation (Jin et al 2012; McGourty et al 2016), the formation of a collagen rich extracellular matrix also appears to be regulated by O-GlcNAcylation of COPII proteins (Cox et al 2018). As discussed above, knockout of Sec23A results in intracellular retention of procollagen.…”