Formation of ER-lumenal intermediates during export of Plasmodium proteins containing transmembrane-like hydrophobic sequences.

During the blood stage of a malaria infection, malaria parasites export both soluble and membrane proteins into the erythrocytes in which they reside. Exported proteins are trafficked via the parasite endoplasmic reticulum and secretory pathway, before being exported across the parasitophorous vacuole membrane into the erythrocyte. Transport across the parasitophorous vacuole membrane requires protein unfolding, and in the case of membrane proteins, extraction from the parasite plasma membrane. We show that trafficking of the exported Plasmodium protein, Pf332, differs from that of canonical eukaryotic soluble-secreted and transmembrane proteins. Pf332 is initially ER-targeted by an internal hydrophobic sequence that unlike a signal peptide, is not proteolytically removed, and unlike a transmembrane segment, does not span the ER membrane. Rather, both termini of the hydrophobic sequence enter the ER lumen and the ER-lumenal species is a productive intermediate for protein export. Furthermore, we show in intact cells, that two other exported membrane proteins, SBP1 and MAHRP2, assume a lumenal topology within the parasite secretory pathway. Although the addition of a C-terminal ER-retention sequence, recognised by the lumenal domain of the KDEL receptor, does not completely block export of SBP1 and MAHRP2, it does enhance their retention in the parasite ER. This indicates that a sub-population of each protein adopts an ER-lumenal state that is an intermediate in the export process. Overall, this suggests that although many exported proteins traverse the parasite secretory pathway as typical soluble or membrane proteins, some exported proteins that are ER-targeted by a transmembrane segment-like, internal, non-cleaved hydrophobic segment, do not integrate into the ER membrane, and form an ER-lumenal species that is a productive export intermediate. This represents a novel means, not seen in typical membrane proteins found in model systems, by which exported transmembrane-like proteins can be targeted and trafficked within the lumen of the secretory pathway.

Calculations on the Ruthenium-Catalyzed Diene and Dienyne Ring-Closing Metathesis Reactions in the Synthesis of Taxol Derivatives.

Density-functional and semiempirical calculations (M06, M06L, and PM6) on intermediates in the ring-closing metathesis (RCM) reactions in the synthesis of Taxol derivatives give results in excellent agreement with the results of previous experimental work. The results suggest that the degree of steric overloading plays a decisive role in determining the outcome (ene-ene or ene-yne-ene metathesis). Due to the rigidity of the Taxol skeleton being formed in the ene-yne-ene cascade reaction, the transition states in its final ene-ene metathesis reaction stage are particularly sensitive to steric effects. Thus, the reaction is predicted to be preferred for one diastereomer of the precursor in which the diol functionality is protected with a compact cyclic carbonate moiety, whereas the use of a bulkier benzoate-protecting group results in activation barriers for Taxol formation that are prohibitive. The reason why one diastereomer of the carbonate-protected precursor undergoes formation of a tricycle via an ene-yne-ene RCM cascade, whereas the other diastereomer undergoes cyclooctene formation via an ene-ene RCM, likely lies in the orientation of the pseudoaxial methyl group on the cyclohexene ring, which in the latter case would unfavorably point toward the reactive center of the Ru-complex, leading to Taxol formation.