Phosphorylation of LCB1 subunit of serine palmitoyltransferase stimulates its activity and modulates sphingolipid biosynthesis in Arabidopsis

Abstract Sphingolipids are the structural elements for membrane lipid bilayers and the signal molecules for many cellular processes. Serine palmitoyltransferase (SPT) is the first committed and rate-limiting enzyme in the de novo sphingolipids biosynthetic pathway. The core SPT was previously suggested as a heterodimer consisting of LCB1 and LCB2 subunits. The SPT activity was shown to be inhibited by orosomucoid proteins (ORMs) and stimulated by small subunits of SPT (ssSPT). However, whether LCB1 is modified and how the modification regulates SPT activity have been unclear. Here, we show that activation of MPK3 and MPK6 by upstream MKK9 and Flg22 (a pathogen-associated molecular pattern) treatment increases SPT activity and induces the accumulation of sphingosine long-chain base (LCB) t18:0 in Arabidopsis thaliana; the activated MPK3 and MPK6 phosphorylate AtLCB1. Phosphorylation of AtLCB1 strengthens its binding with AtLCB2b, promotes its binding with ssSPTs, and stimulates the higher-order-oligomer and active SPT complexes formation. Our findings suggest a novel regulatory mechanism of SPT activity.