LIG_RuBisCO_WRxxL_1

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase; P00875) is the most prevalent enzyme on earth. It is responsible for the carboxylation of ribulose-1,5-bisphosphate, the first step in the carbon fixation process. The RuBisCO complex is a 16mer (Heterohexadecamer) with 8 copies of the catalytic large subunit and 8 copies of the small subunit. RuBisCO enzymes possess a slow catalytic rate, but plants have overcome this by evolving different CO2 concentrating mechanisms to increase RuBisCO performance. One such mechanism is formation of the Pyrenoid, a membrane-less compartment mainly composed of RuBisCO within the stroma of chloroplasts of some algae such as Chlamydomonas. Additional structures are formed at the Pyrenoid to concentrate CO2, such as the traversing tubules and a starch sheath, in limiting CO2 conditions (Wang,2015).
EPYC1 (Essential Pyrenoid Component 1; A0A2K3DA85) is another abundant protein in the Pyrenoid. EPYC1 is a ~35 kDa intrinsically disordered protein that has been shown to phase separate with RuBisCO and form liquid droplets in vitro (Wunder,2018). Meyer,2020 determined that the WRxxL motif was the element by which EPYC1 and the RuBisCO small subunit (RBCS) interact. EPYC1 contains multiple copies of this motif and is able to bind to any of the 8 RuBisCO’s RBCSs. Motif instances are spaced far enough away from each other to allow EPYC1 to interact with multiple RuBisCO complexes (He,2020).
Other less abundant pyrenoid proteins also contain instances of the WRxxL motif. Furthermore, they also possess additional protein domains for binding to structured assemblies of the Pyrenoid: SAGA1 & SAGA2 (A0A2K3D7T6;A0A2K3DEH1) for the starch sheath and RBMP1, RBMP2 (A0A2K3DMS8;A0A2K3DG19) for the traversing tubules. Other chloroplast non-pyrenoid proteins can be targeted to the Pyrenoid by adding the motif, which would allow them to bind to the RBSC. In Meyer,2020 the authors propose a role for the WRxxL motif as the organiser of the Pyrenoid’s architecture. First by allowing the multivalent EPYC1:RuBisCO interaction which would maintain the Pyrenoid matrix, and then other motif-containing proteins would dock to the Pyrenoid and assemble the extra structures using specific interaction domains (Meyer,2020).