Arabidopsis Squalene Epoxidase 3 (SQE3) Complements SQE1 and is important for embryo development and bulk squalene epoxidase activity

The existence of multigenic families in the mevalonate pathway suggests divergent functional roles for pathway components involved in the biosynthesis of plant sterols. Squalene epoxidases (SQEs) are key components of this pathway, and Squalene Epoxidase 1 (SQE1) has been identified as a fundamental enzyme in this biosynthetic step. In the present work, we extended the characterization of the remaining SQE family members, phylogenetically resolving between true SQEs and a subfamily of SQE-like proteins that is exclusive to Brassicaceae. Functional characterization of true SQE family members, Squalene Epoxidase 2 (SQE2) and Squalene Epoxidase 3 (SQE3), indicates that SQE3, but not SQE2, contributes to the bulk SQE activity in Arabidopsis, with sqe3-1 mutants accumulating squalene and displaying sensitivity to terbinafine. We genetically demonstrated that SQE3 seems to play a particularly significant role in embryo development. Also, SQE1 and SQE3 both localize in the endoplasmic reticulum, and SQE3 can functionally complement SQE1. Thus, SQE1 and SQE3 seem to be two functionally unequal redundant genes in the promotion of plant SQE activity in Arabidopsis.

This work was supported by Ministerio de Ciencia e Innovacio´ n, cofinanced by the European Regional Development Fund (BIO2011-23859 to M.A.B.). This work was also supported by Fundac¸ a˜ o para a Ciencia e ^ Tecnologia (SFRH/BD/29778/2006 to S.L.). H.A. was supported by the Genomics and Evolutionary Biology project, co-financed by North Portugal Regional Operational Programme 2007/2013 (ON.2 – O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF). A.R. was supported by the NSERC Discovery Grant (RGPIN-2014-06468) and funds from the Canada Research Chair.

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