Michal Pyc, John Dyer and Robert Mullen, Department of Molecular and Cellular Biology, University of Guelph, Guelph, Canada, U.S. Department of Agriculture, Agricultural Research Service, U.S. Arid-Land Agricultural Research Center, Maricopa, AZ, USA
Background: The storage and use of fats are critical in many cellular and developmental processes in plants. While fat storage is typically associated with seeds, all plant cells produce and package fats into organelles termed lipid droplets (LDs). Oleosins have been classically associated with LD formation in seeds, but the roles of additional LD ‘coat’ proteins, particularly in non-seed tissues lacking oleosins, are far less defined. LDs are produced from the endoplasmic reticulum (ER) and, in recent years, characterization of ER-localized SEIPINs and the LD-associated proteins (LDAPs) have begun to shed light on additional proteins involved in LD biogenesis. Among these, the LDAP-interacting protein (LDIP) was shown to be important for regulating LD size and number, but how this protein functions was unclear.
Question: How does LDIP function together with other components of the LD biogenesis machinery to support proper LD formation?
Findings: LDIP was initially identified as an LD coat protein, but preliminary protein–protein association studies suggested a physical and/or functional interaction with both ER-localized SEIPINs and LD-localized LDAPs. We found that LDIP regulates leaf LD size and number independent of the LDAPs. Additionally, the targeting of LDIP to LDs is dependent on localization of LDAP to LDs. We also found that LDIP interacts with SEIPINs and both proteins work together to regulate LD size and number. Taken together, our work suggests a step-wise process of protein interactions between SEIPINs, LDIP and LDAPs at sites of LD formation in the ER, and that also draws intriguing parallels to models of LD biogenesis in mammals.
Next steps: Our future efforts will be focused on the molecular mechanisms by which these proteins, as well as others, such as oleosins, regulate the growth and composition of LDs as they emerge from the surface of the ER. Namely, what are the very early steps in LD formation and how are lipids transferred from the ER to the growing LD?
Michal Pyc, Satinder K. Gidda, Damien Seay, Nicolas Esnay, Franziska K. Kretzschmar, Yingqi Cai, Nathan M. Doner, Michael S. Greer, J. Joe Hull, Denis Coulon, Claire Bréhélin, Olga Yurchenko, Jan de Vries, Oliver Valerius, Gerhard H. Braus, Till Ischebeck, Kent D. Chapman, John M. Dyer, Robert T. Mullen. (2021). LDIP Cooperates with SEIPIN and LDAP to Facilitate Lipid Droplet Biogenesis in Arabidopsis. Plant Cell. https://doi.org/10.1093/plcell/koab179