Concomitant post-translational repression of Arabidopsis PIP1 aquaporins upon the loss of major PIP2 isoforms

Aquaporins at the plant plasmalemma are divided into two highly conserved subclasses, PLASMA MEMBRANE INTINSIC PROTEINs 1 (PIP1) and PIP2. Arabidopsis thaliana encodes five PIP1 and eight PIP2 isoforms. Individual loss-of-function mutants had been employed for functional analyses. Here, we observe that the pip2;1 pip2;2 pip2;4 pip2;6 pip2;7 quintuple mutant defective of major PIP2 isoforms concomitantly leads to a strongly reduced PIP1 protein level. Lower order mutants pip2;1 pip2;2 and pip2;1 pip2;2 pip2;7 still harbor only 60% and 20% residual PIP1, respectively. This repression is established post-translationally, since neither PIP1s steady-state transcripts nor polysome-associated PIP1 mRNAs are suppressed by pip2;1 pip2;2 pip2;7. Thus, the two major pathways operating in eukaryotes for removal of aberrant proteins, ubiquitin proteasome system (UPS)-dependent ER-associated degradation (ERAD) and autophagy/vacuole-linked degradation, were assessed. Introgression of atg7 blocking autophagy-mediated degradation does not affect the PIP1 protein level of pip2;1 pip2;2 pip2;7. In contrast, introgression of ERAD loss-of-function mutations hrd1A hrd1B and dln1 into pip2;1 pip2;2 pip2;7 partially stabilizes its PIP1 protein level. PIP1 accumulates intracellularly upon pharmacological inhibition of proteasomal degradation by MG132. Nevertheless, the lack of a full PIP1 recovery by these means suggests the flexible operation of parallel ERAD components or unknown pathways. In conclusion, the essential dependence of PIP1 expression on PIP2 isoforms intrinsically interconnects the two PIP subclades at the protein level and will thereby affect their mutual functions. Significance statementPlasma membrane intrinsic proteins constituting the most homogenous plant aquaporin family are nonetheless split into two highly conserved subfamilies, PIP1 and PIP2. The loss of major Arabidopsis PIP2 isoforms does not lead to compensation by PIP1 members, but rather to PIP1s concomitant, post-translational repression. This dependence of PIP1 isoforms inevitably ties the two PIP subfamilies and their function.