other lysosomal membrane proteins are
not only N-glycosylated, but also contain several O-linked
glycans which turn out to play a similar role for the regulated
release of content (Wilke et al., 2012; Li et al., 2015).
Despite the fact that only N-glycosylation was investigated
in the study from Pedrazzini and colleagues (2016), it is
unlikely that other types of protein glycosylation safeguard
tonoplast proteins from degradation by vacuolar hydrolases.
O-glycosylation, the second major type of protein glycosylation,
is fundamentally different between mammals and
plants and the whole machinery for mucin-type mammalian
O-glycosylation is not present in plants (Strasser, 2012).
Plant-specific modifications of O-glycosylation sites have
been described, but the attached glycan moieties are mainly
present on cell wall proteins and typically not found on vacuolar
proteins (Tan et al., 2012). Consequently, it seems that
a similar N- or O-glycan shield does not contribute to the
protection of tonoplast proteins from intracellular proteolysis
in plants.