Linker domain Ser-phosphorylation as a putative modulator for ABCB alkaloid transporters

In silico analysis of different ABCB-type proteins revealed structural similarities within the linker domain, including the presence of [R/K-X-S/T] and [R/K-X-X-S/T] motifs, which are potential phosphorylation sites for kinases of the AGCVIII superfamily. Based on these findings, a phosphorylation-based regulatory mechanism is proposed for ABCB alkaloid transporters. Plants avoid self-toxicity by compartmentalizing secondary metabolites in specialized cells. Even when this mechanism keeps them away from sensitive metabolic processes, it also could restrict them from reaching the different locations where they might be required. Hence, operation of transport mechanisms is critical to fulfill their assigned functions. Posttranslational modifications offer a versatile mechanism for the fine tuning of protein functions, which has been marginally explored in plant ABC transporters. The activation of camalexin transport by serine phosphorylation of Arabidopsis PDR8/ABCG36 is one of the few documented examples. However, similar mechanisms have been observed in examples from different taxa, such as the human HsABCB1/MDR1/P-gp (ABCB), Saccharomyces ScYCF1 (ABCC), and Candida CaCdr1p (ABCG), among others. ABC proteins include domains for both membrane anchoring and nucleotide hydrolysis. In some instances, these domains are joined by an additional linker domain in a single polypeptide. Even when the linker domain is mostly regarded as a structural spacer, in some ABCB transporters, the presence of serine residues, potentially recognizable by protein kinases from the AGC superfamily, suggests a possible modulatory role. In fact, Ser phosphorylation on the linker domain of the human HsABCB1 modifies its selectivity against different allocrites, whereas in the Arabidopsis transporter AtABCB1 linker domain phosphorylation increases cell efflux of indoleacetic acid. ABCB transporters for benzylisoquinoline alkaloids have been described in different plants and the occurrence of Ser residues in the right phosphorylation context suggests a possible extension of this regulatory mechanism to secondary metabolite transporters.