A Sec14 domain protein is required for photoautotrophic growth and chloroplast vesicle formation in Arabidopsis thaliana

Abstract

In eukaryotic photosynthetic organisms, the conversion of solar into chemical energy occurs in thylakoid membranes in the chloroplast. How thylakoid membranes are formed and maintained is poorly understood. However, previous observations of vesicles adjacent to the stromal side of the inner envelope membrane of the chloroplast suggest a possible role of membrane transport via vesicle trafficking from the inner envelope to the thylakoids. Here we show that the model plant Arabidopsis thaliana has a chloroplast-localized Sec14-like protein (CPSFL1) that is necessary for photoautotrophic growth and vesicle formation at the inner envelope membrane of the chloroplast. The cpsfl1 mutants are seedling lethal, show a defect in thylakoid structure, and lack chloroplast vesicles. Sec14 domain proteins are found only in eukaryotes and have been well characterized in yeast, where they regulate vesicle budding at the trans-Golgi network. Like the yeast Sec14p, CPSFL1 binds phosphatidylinositol phosphates (PIPs) and phosphatidic acid (PA) and acts as a phosphatidylinositol transfer protein in vitro, and expression of Arabidopsis CPSFL1 can complement the yeast sec14 mutation. CPSFL1 can transfer PIP into PA-rich membrane bilayers in vitro, suggesting that CPSFL1 potentially facilitates vesicle formation by trafficking PA and/or PIP, known regulators of membrane trafficking between organellar subcompartments. These results underscore the role of vesicles in thylakoid biogenesis and/or maintenance. CPSFL1 appears to be an example of a eukaryotic cytosolic protein that has been coopted for a function in the chloroplast, an organelle derived from endosymbiosis of a cyanobacterium.

Chloroplast Sec14-like 1 (CPSFL1) is essential for normal chloroplast development and affects carotenoid accumulation in Chlamydomonas

“…Alternatively, CPSFL1 might be involved in a vesicle transport pathway of carotenoids from the chloroplast envelope (57). Such a function would be consistent with our results on the cpsfl1 mutant of A. thaliana, which lacks chloroplast vesicles that are detectable in the WT (58). A defect in carotenoid transport could result in an accumulation of carotenoids or biosynthetic intermediates in the envelope, which could cause feedback inhibition of carotenoid biosynthesis as described above.…”

Phosphoinositides regulate chloroplast processes

“…Based on these results, Hertle et al (12) propose that CPSFL1 plays a role in chloroplast vesicle formation, in line with a role of yeast Sec14 in membrane trafficking through late Golgi/ endosomal compartments. Since no vesicles were formed in the mutant but thylakoids still were formed, vesicle traffic appears not to be required for the biogenesis of thylakoid membranes.…”

“…Members of two subfamilies adopt a multidomain structure in which the N-terminal Sec14 domain is linked to a C-terminal Nlj16-like "nodulin" or a GOLD (Golgi dynamics) domain, while members of a third subfamily are singledomain proteins. In PNAS, Hertle et al (12) report on a chloroplast-targeted, single-domain Sec14-like protein termed CPSFL1 (chloroplast-localized Sec14-like protein 1) from Arabidopsis. The cpsfl1 knockout mutant was incapable of photoautotrophic growth, which correlated with a strongly reduced accumulation of all photosynthetic complexes in the thylakoid membranes (less than 25% of wild-type levels).…”

“…Chlamydomonas chloroplast VIPP1 and its paralog VIPP2 bind to PIs and VIPP1 rods can engulf PI-containing liposomes in vitro (15,16). Since cyanobacteria possess neither PtdIns (21) nor Sec14-like proteins (12), the adaptation of VIPP1 to PI signaling appears to have evolved during domestication of the cyanobacterial endosymbiont. VIPP1 was proposed to organize membrane nanodomains resembling eisosomes (22).…”

A brief history of how microscopic studies led to the elucidation of the 3D architecture and macromolecular organization of higher plant thylakoids

“…a From Paolillo and Falk (1966); b From Staehelin (1986). a, b Bars 0.5 mm 1 3 to the plasma membrane, while their marginal contact sites become concentrated in the chloroplast poles (Liang et al 2018;Hertle et al 2020). Functionally, the thylakoid-IEM contact sites appear to be responsible for transferring uncharged galactolipids (mono-and di-galactosyldiacylglycerols) from their site of synthesis, the chloroplast envelope membranes (Kobayashi et al 2007), to the thylakoids.…”