br Learning from our ancestors As mentioned earlier homologu
Learning from our ancestors As mentioned earlier, homologues of all γ-secretase components have been identified in plants and protozoans, some of which have emerged as model systems for the study of γ-secretase independent functions of the presenilins . The moss P. patens was the first plant species in which conservation of γ-secretase complex components and a γ-secretase independent function for the presenilin homologue (PpPS) and nicastrin was demonstrated . A more recent study in A. thaliana similarly showed the conservation of γ-secretase complex components in another plant species , . Importantly, certain highly conserved amino GSK1016790A motifs crucial for γ-secretase proteolytic activity, substrate recognition and complex assembly in mammals are similarly conserved in plant species . However, phylogenetic analysis of presenilins indicates that plant and animal homologues fall into two divergent clades . Interestingly, P. patens genome contains only one copy of presenilin (PpPS) and does not possess homologues of several mammalian γ-secretase substrates including APP and Notch . In reconstitution studies PpPS was defective in cleavage of a Notch1-based substrate in PS-deficient mouse embryonic fibroblasts (MEFs), but was able to restore normal proliferation rates in PS-deficient MEFs, a function thought to be independent of γ-secretase proteolytic activity . In the phenotypic characterization of a null mutant of presenilin (Ppps), P. patens displayed abnormal growth pattern, impaired chloroplast movement and decreased endocytosis . In this regard, in Arabidopsis studies employing exogenous expression of γ-secretase subunits impairment of vacuole trafficking was observed . Together this data from two plant species intimates that the predominant function of presenilin in plants is not related to γ-secretase protease activity but perhaps points to an ancestral role for presenilins and the γ-secretase complex in protein endocytosis and trafficking. Consistent with this hypothesis, γ-secretase is present and active in the slime mould D. discoideum,  where the presenilins have been shown to be essential for Dictyostelium cell fate determination and the regulation of phagocytosis. However, in contrast to plants, Dictyostelium γ-secretase was still proteolytically active against animal substrates . Again, this suggests an ancestral role for presenilins that extends well beyond what is known about γ-secretase protease activity from predominantly animal studies. Nevertheless, from animal studies there is also a growing body of evidence showing that the presenilins and γ-secretase may be involved in the regulation of endocytosis, protein trafficking and degradation , , , leading to the proposal that signalling functions associated with γ-secretase proteolytic activity and substrate cleavage arose later in evolution , . In keeping with this, while there are over 90 reported γ-secretase substrates no biological function has been attributed to the majority of these γ-secretase-generated ICDs , . However, many substrate-derived ICDs are rapidly degraded and support the proposal that the predominant function attributed to γ-secretase would be to facilitate the removal and degradation of membrane bound protein fragments subsequent to ectodomain shedding, thereby acting as a ‘membrane proteasome’ . Furthermore, mice deficient in Notch signalling or deficient in either of the γ-secretase components, Aph-1, Pen-2 or Nicastrin, are able to generate anterior somites, whereas Psen1 and Psen2 double-deficient animals are not, suggesting that the presenilins contribute to the generation of anterior somites, independent of their role in γ-secretase . In summary, from an evolutionary perspective, data supports the proposal that presenilins have an important regulatory role in protein endocytosis, trafficking and degradation, which preceded signalling functions associated with γ-secretase substrate cleavage and proteolytic activities.