LIP4 Is Involved in Triacylglycerol Degradation in $Chlamydomonas\ reinhardtii$
Résumé
Degradation of the storage compound triacylglycerol (TAG) is a crucial process in response to environmental stimuli. Failing to respond properly may be detrimental for survival. In microalgae, this process is important for re-growth when conditions become favorable after cells have experienced stresses. Mobilization of TAG is initiated by actions of lipases causing the release of glycerol and free fatty acids, which can be further broken down for energy production or recycled to synthesize membrane lipids. Although key enzymes in the process, TAG lipases remain to be characterized in the model green alga $Chlamydomonas\ reinhardtii$. Here we describe the functional analysis of a putative TAG lipase, i.e. CrLIP4, which shares 44% amino acid identity with the major TAG lipase in $Arabidopsis$ (SUGAR DEPENDENT1- SDP1). The $CrLIP4$ transcript level was down regulated during nitrogen deprivation (ND) when TAG accumulates, but was upregulated during nitrogen recovery (NR) when TAG was
degraded. Both artificial microRNA knockdown and insertional knockout mutants showed a delay in TAG mobilization during NR. The difference in TAG degradation was more pronounced when the cultures were incubated without acetate in the dark. Furthermore, the $crlip4$ knock-out mutant over-accumulated TAG during optimal growth conditions. Taken together, the results suggest to us that CrLIP4 likely acts as a TAG lipase and plays a major role in TAG homeostasis in $Chlamydomonas$.
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