Novel Inhibitory Function of the Rhizomucor miehei Lipase Propeptide and Three-Dimensional Structures of Its Complexes with the Enzyme.

Many proteins are synthesized as precursors, with propeptides playing a variety of roles such as assisting in folding or preventing them from being active within the cell. While the precise role of the propeptide in fungal lipases is not completely understood, it was previously reported that mutations in the propeptide region of the Rhizomucor miehei lipase have an influence on the activity of the mature enzyme, stressing the importance of the amino acid composition of this region. We here report two structures of this enzyme in complex with its propeptide, which suggests that the latter plays a role in the correct maturation of the enzyme. Most importantly, we demonstrate that the propeptide shows inhibition of lipase activity in standard lipase assays and propose that an important role of the propeptide is to ensure that the enzyme is not active during its expression pathway in the original host.

Structure and dynamics of photosystem II light-harvesting complex revealed by high-resolution FTICR mass spectrometric proteome analysis.

Structure and dynamics of membrane-bound light-harvesting pigment-protein complexes (LHCs), which collect and transmit light energy for photosynthesis and thereby play an essential role in the regulation of photosynthesis and photoprotection, were identified and characterized using high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). LHCs from photosystem II (LHCII) were isolated from the thylakoid membrane of Arabidopsis thaliana leaves after light stress treatment using sucrose density gradient centrifugation, and separated by gel-filtration into LHCII subcomplexes. Using reversed-phase high-performance liquid chromatography and two-dimensional (2D) gel electrophoresis, the LHCII proteins, Lhcb1-6 and fibrillins, were efficiently separated and identified by FTICR-MS. Some of the LHCII subcomplexes were shown to migrate from photosystem II to photosystem I as a result of short-term adaptation to changes in light intensity. In the mobile LHCII subcomplexes, decreased levels of fibrillins and a modified composition of LHCII protein isoforms were identified compared to the tightly bound LHCII subcomplexes. In addition, FTICR-MS analysis revealed several oxidative modifications of LHCII proteins. A number of protein spots in 2D gels were found to contain a mixture of proteins, illustrating the feasibility of high-resolution mass spectrometry to identify proteins that remain unseparated in 2D gels even upon extended pH gradients.

Differential expression and localization of early light-induced proteins in Arabidopsis.

The early light-induced proteins (Elips) in higher plants are nuclear-encoded, light stress-induced proteins located in thylakoid membranes and related to light-harvesting chlorophyll (LHC) a/b-binding proteins. A photoprotective function was proposed for Elips. Here we showed that after 2 h exposure of Arabidopsis (Arabidopsis thaliana) leaves to light stress Elip1 and Elip2 coisolate equally with monomeric (mLhcb) and trimeric (tLhcb) populations of the major LHC from photosystem II (PSII) as based on the Elip:Lhcb protein ratio. A longer exposure to light stress resulted in increased amounts of Elips in tLhcb as compared to mLhcb, due to a reduction of tLhcb amounts. We demonstrated further that the expression of Elip1 and Elip2 transcripts was differentially regulated in green leaves exposed to light stress. The accumulation of Elip1 transcripts and proteins increased almost linearly with increasing light intensities and correlated with the degree of photoinactivation and photodamage of PSII reaction centers. A stepwise accumulation of Elip2 was induced when 40% of PSII reaction centers became photodamaged. The differential expression of Elip1 and Elip2 occurred also in light stress-preadapted or senescent leaves exposed to light stress but there was a lack of correlation between transcript and protein accumulation. Also in this system the accumulation of Elip1 but not Elip2 correlated with the degree of PSII photodamage. Based on pigment analysis, measurements of PSII activity, and assays of the oxidation status of proteins we propose that the discrepancy between amounts of Elip transcripts and proteins in light stress-preadapted or senescent leaves is related to a presence of photoprotective anthocyanins or to lower chlorophyll availability, respectively.

Covalent binding of chloroacetamide herbicides to the active site cysteine of plant type III polyketide synthases.

Chloroacetamide herbicides inhibit very-long-chain fatty acid elongase, and it has been suggested that covalent binding to the active site cysteine of the condensing enzyme is responsible [Pest Manage Sci 56 (2000), 497], but direct evidence was not available. The proposal implied that other condensing enzymes might also be targets, and therefore we have investigated four purified recombinant type III plant polyketide synthases. Chalcone synthase (CHS) revealed a high sensitivity to the chloroacetamide metazachlor, with 50% inhibition after a 10 min pre-incubation with 1-2 molecules per enzyme subunit, and the inactivation was irreversible. Stilbene synthase (STS) inactivation required 20-fold higher amounts, and 4-coumaroyltriacetic acid synthase and pyrone synthase revealed no response at the highest metazachlor concentrations tested. A similar spectrum of differential responses was detected with other herbicides that also inhibit fatty acid elongase (metolachlor and cafenstrole). The data indicate that type III polyketide synthases are potential targets of these herbicides, but each combination has to be investigated individually. The interaction of metazachlor with CHS was investigated by mass spectrometric peptide mapping, after incubation of the enzymes with the herbicides followed by tryptic digestion. A characteristic mass shift and MS/MS sequencing of the respective peptide showed that metazachlor was covalently bound to the cysteine of the active site, and the same was found with STS. This is the first direct evidence that the active site cysteine in condensing enzymes is the primary common target of these herbicides.