Evolution of vacuolar targeting in algae

Sorting of soluble vacuolar proteins destined for vacuoles is facilitated by vacuolar sorting receptors (VSRs). To date, three different types of VSRs have been described, namely the mammalian mannose-6-phosphate receptor, the yeast VPS10p receptor, and a plant-type receptor. We searched completed algal genomes for the presence of the plant-type VSRs and examined their structure and phylogeny. We show that green algae and some chromalveolates contain a plant-type vacuolar receptor that seems to be absent in glaucophytes and rhodophytes. All putative plant-type protistan VSRs examined have a similar protein structure. The results of a phylogenetic analysis suggest that plant-type VSRs may have evolved in the last common ancestor of the Viridiplantae and that chromalveolates obtained this type of VSR by horizontal or endocytotic gene transfer. Gene duplications took place in several viridiplant and chromalveolate lineages. We present hypotheses about the evolution of paralogous plant-type VSRs in chlorophytes and chromalveolates.