Arabidopsis NAP1 Regulates the Formation of Autophagosomes

The SCAR/WAVE complex is required for ARP2/3-mediated actin nucleation, and these complexes are highly conserved in plants and animals [1Davidson A.J. Insall R.H. Actin-based motility: WAVE regulatory complex structure reopens old SCARs.Curr. Biol. 2011; 21: R66-R68Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar, 2Deeks M.J. Hussey P.J. Arp2/3 and SCAR: plants move to the fore.Nat. Rev. Mol. Cell Biol. 2005; 6: 954-964Crossref PubMed Scopus (58) Google Scholar]. Proteins from the SCAR/WAVE complex have been found to be membrane associated in plants [3Dyachok J. Shao M.R. Vaughn K. Bowling A. Facette M. Djakovic S. Clark L. Smith L. Plasma membrane-associated SCAR complex subunits promote cortical F-actin accumulation and normal growth characteristics in Arabidopsis roots.Mol. Plant. 2008; 1: 990-1006Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar]. Using fluorescent protein fusions, we have found that NAP1 [4Deeks M.J. Kaloriti D. Davies B. Malhó R. Hussey P.J. Arabidopsis NAP1 is essential for Arp2/3-dependent trichome morphogenesis.Curr. Biol. 2004; 14: 1410-1414Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar], a component of the SCAR/WAVE complex, locates to vesicles or puncta that appear upon applied pressure. These NAP1 vesicles can be endoplasmic reticulum (ER)-associated, can co-align with the cytoskeleton, and fuse to each other homotypically. More interestingly, the majority co-localizes with the autophagosome marker ATG8, and anti-NAP1 identifies autophagosomes in immuno-TEM. Macroautophagy (hereafter referred to as autophagy) is enhanced under certain stress conditions such as nitrogen starvation and salt stress. We show that fewer autophagosomes are generated in the NAP1 knockout mutant during starvation stress. The nap1 mutant (and KO mutants of other components of the SCAR/WAVE and ARP2/3 complexes) is more susceptible to nitrogen starvation and is less salt tolerant, indicating defective autophagy. In conclusion, our data show that NAP1 has another function in plant cells, and that is as a regulator of autophagy.