Uncertain role of MtSEO-F3 in assembly ofMedicago truncatulaforisomes

Abstract

Forisome structures are fine-tuned by the subunit composition Forisomes are large, spindle-shaped protein complexes that are present in the sieve elements of papilionoid legumes.1,2 In their spindle-shaped conformation, forisomes facilitate the transport of photoassimilates along the sieve tube lumen, but when the vascular system is wounded they change into a pluglike/expanded conformation that causes occlusion to prevent leakage. After tissue regeneration, forisomes can revert to their spindle-shaped conformation, thereby providing a reversible sieve tube plugging mechanism. The conformational change is calcium-dependent and can also be induced ex vivo by nonphysiological pH values.3 The forisome subunits in M. truncatula were identified by mass spectrometry following the purification of native forisomes. 4 The MtSEO-F1 protein sequence (formerly Mtfor1) was used for homology searches and transcriptional analysis, resulting in the characterization of a large sieve element occlusion (SEO) protein family spanning a wide range of dicot plants. 5,6 The group 1 SEO proteins are only present in papilionoid legumes and these represent forisome subunits, whereas members of groups 5 and 6 represent the common P-proteins in most dicot species that probably cause irreversible sieve element occlusion. [5][6][7][8] In our recent study, we expressed group 1 SEO-F genes from different legumes in a heterologous background to study their impact on forisome assembly, including the DpSEO-F1 gene from the basal papilionoid species D. panamensis. 9 The D. panamensis lineage diverged from lineages of the Old World clade, i.e., Canavalia gladiata, Lotus japonicus, and Medicago truncatula, more than 50 million years ago. The artificial protein bodies produced by expressing SEO-F genes from Lotus japonicus , , Vicia faba (VfSEO-F1), and M. truncatula ) appeared longer and thinner than wild-type forisomes, whereas the artificial forisome composed of DpSEO-F1 was similar in geometry to its native counterpart. We also found that forisome geometry was affected by coexpression, e.g., MtSEO-F2 was able to widen the forisome body Forisomes are specialized multimeric protein complexes found only in the papilionoid legumes. they undergo a reversible conformational change in response to phloem injury to enable the occlusion of sieve tubes, thus preventing the loss of photoassimilates. the individual subunits are designated by the letters SEo-F (sieve element occlusion by forisomes) and are part of the larger SEo protein family, which also includes the typical P-proteins found in most dicots and some monocots. When specific SEo-F subunits from different species are expressed in a heterologous background, they self-assemble into fully-functional artificial forisomes. however, with the exception of basal species such as Dipteryx panamensis, the geometry of these artificial forisomes differs from that of their native counterparts. Studies involving SEo-F proteins from the model legume Medicago truncatula have shown that a combination of 3 of the 4 ...

Guardians of the phloem – forisomes and beyond

“…Artificial forisomes consisting of all three subunits (MtSEO‐F1, MtSEO‐F2 and MtSEO‐F4) are most similar to natural forisomes in geometry and reaction (Müller et al ., 2014). The function of MtSEO‐F3 as a forisome subunit has yet to be confirmed (Groscurth et al ., 2014). The physiological advantage of several partly redundant forisome subunits is not known.…”

Forizymes – functionalised artificial forisomes as a platform for the production and immobilisation of single enzymes and multi-enzyme complexes

“…These subunits appear to be primarily responsible for the formation of the native forisome body, whereas MtSEO-F2 appears to fine-tune the geometric proportions of forisomes and hence their activity 25 . The function of MtSEO-F3 remains unclear 26 . Artificial forisomes produced by expressing MtSEO-F1 or MtSEO-F4 in the yeast Saccharomyces cerevisiae also self-assemble into micro-scale forisome structures even when small additional tags are translationally fused to the forisome subunits 24 27 .…”