We analyze the potential for the spin manipulation of vertical-cavity surface-emitting lasers (VCSELs) by operating them electrically and injecting additional spin-polarized carriers by polarized optical excitation. The output polarization of the VCSELs can be easily controlled by the spin orientation of the optically injected carriers when the injection current does not exceed the threshold current

Key proteins of the photosynthetic complexes are encoded in the chloroplast genome and cotranslationally inserted into the thylakoid membrane. However, the molecular details of this process are largely unknown. Here, we demonstrate by ribosome profiling that the conserved chloroplast signal recognition particle subunit (cpSRP54) is required for efficient cotranslational targeting of several central photosynthetic proteins, such as the PSII PsbA (D1) subunit, in Arabidopsis (Arabidopsis thaliana). High-resolution analysis of membrane-associated and soluble ribosome footprints revealed that the SRP-dependent membrane targeting of PsbA is already initiated at an early translation step before exposure of the nascent chain from the ribosome. In contrast to cytosolic SRP, which contacts the ribosome close to the peptide tunnel exit site, analysis of the cpSRP54/ribosome binding interface revealed a direct interaction of cpSRP54 and the ribosomal subunit uL4, which is not located at the tunnel exit site but forms a part of the internal peptide tunnel wall by a loop domain. The plastid-specific C-terminal tail region of cpSRP54 plays a crucial role in uL4 binding. Our data indicate a novel mechanism of SRP-dependent membrane protein transport with the cpSRP54/uL4 interaction as a central element in early initiation of cotranslational membrane targeting.

The synthesis of multi-span thylakoid membrane proteins initiates at ribosomes off the membrane. Subsequently, the ribosome nascent chain complexes (RNCs) are transferred to the translocase machinery in the thylakoid membrane for cotranslational protein insertion. These steps require finely tuned mechanisms for protein processing, quality control, and targeting to prevent misfolding or aggregation and to ensure efficient transfer of the nascent chain to the insertion machinery. However, little is known about the regulatory network underlying these processes. To identify factors specifically involved in the cotranslational biogenesis of the reaction center protein D1 of photosystem II we established a chloroplast-derived in vitro translation method that allows the production and affinity purification of stalled RNCs bearing nascent chains of D1 of different defined lengths. Stalled RNCs translating the soluble ribosomal subunit uS2c were affinity-purified for comparison. Quantitative tandem-mass spectrometry revealed a set of about 120 proteins specifically associated with D1 RNCs. The interactome includes proteins with broad functions in protein processing, biogenesis and metabolic pathways, such as chlorophyll biosynthesis. We identified STIC2 as a new factor specifically associated with D1 RNCs. Furthermore, our results demonstrated that the interaction of STIC2 with the thylakoid insertase Alb3 and its homologue Alb4 is mediated by the conserved motif III within the C-terminal regions of Alb3 and Alb4. Our data suggest that STIC2 is involved in cotranslational substrate delivery at the thylakoid membrane by coordinating the binding of the D1 RNCs to the insertase machinery.