The reports of dual-targeted proteins in plants have steadily increased over
the past years. The vast majority of these proteins are soluble proteins
distributed between compartments of the non-secretory pathway, predominantly
chloroplasts and mitochondria. In contrast, dual-targeted transmembrane
proteins, especially of the secretory pathway, are rare and the mechanisms
leading to their differential targeting remain largely unknown. Here, we
report dual-targeting of the Arabidopsis DUF679 Membrane Protein 1 (DMP1) to
the tonoplast (TP) and the plasma membrane (PM). In Arabidopsis and tobacco
two equally abundant DMP1 isoforms are synthesized by alternative translation
initiation: a full length protein, DMP1.1, and a truncated one, DMP1.2, which
lacks the N-terminal 19 amino acids including a TP-targeting dileucine motif.
Accumulation of DMP1.1 and DMP1.2 in the TP and the PM, respectively, is
Brefeldin A-sensitive, indicating transit via the Golgi. However, DMP1.2
interacts with DMP1.1, leading to extensive rerouting of DMP1.2 to the TP and
“eclipsed” localization of DMP1.2 in the PM where it is barely visible by
confocal laser scanning microscopy but clearly detectable by membrane
fractionation. It is demonstrated that eGFP fusion to either DMP1 terminus can
cause mistargeting artifacts: C-terminal fusion to DMP1.1 or DMP1.2 results in
altered ER export and N-terminal fusion to DMP1.1 causes mistargeting to the
PM, presumably by masking of the TP targeting signal. These results illustrate
how the interplay of alternative translation initiation, presence or absence
of targeting information and rerouting due to protein-protein interaction
determines the ultimate distribution of a transmembrane protein between two
membranes