SYNOPSISWater-soluble, random copolymers containing N 5-(4-hydroxybutyl ) -L-glutamine (host) and S-methylthio-L-cysteine (guest) have been prepared, fractionated, and characterized, with S-methylthio-L-cysteine serving as a model for cystine residues in proteins. From the thermally induced helix-coil transition curves of these copolymers in water a t neutral pH, the Zimm-Bragg parameters CJ and s for the helix-coil transition of "poly (L-cystine)" were deduced. The results show that the cystine model acts as a weak helix-breaker over the entire temperature range from 0 to 60°C. The implications of this finding are evaluated in the context of a general discussion of the Zimm-Rragg parameters for all the 20 naturally occurring amino acids

Plants accumulate a family of hydrophobic polymers known as polyprenols, yet how they are synthesized, where they reside in the cell, and what role they serve is largely unknown. Using Arabidopsis thaliana as a model, we present evidence for the involvement of a plastidial cis-prenyltransferase (AtCPT7) in polyprenol synthesis. Gene inactivation and RNAi-mediated knockdown of AtCPT7 eliminated leaf polyprenols, while its overexpression increased their content. Complementation tests in the polyprenol-deficient yeast Δrer2 mutant and enzyme assays with recombinant AtCPT7 confirmed that the enzyme synthesizes polyprenols of ;55 carbons in length using geranylgeranyl diphosphate (GGPP) and isopentenyl diphosphate as substrates. Immunodetection and in vivo localization of AtCPT7 fluorescent protein fusions showed that AtCPT7 resides in the stroma of mesophyll chloroplasts. The enzymatic products of AtCPT7 accumulate in thylakoid membranes, and in their absence, thylakoids adopt an increasingly "fluid membrane" state. Chlorophyll fluorescence measurements from the leaves of polyprenol-deficient plants revealed impaired photosystem II operating efficiency, and their thylakoids exhibited a decreased rate of electron transport. These results establish that (1) plastidial AtCPT7 extends the length of GGPP to ;55 carbons, which then accumulate in thylakoid membranes; and (2) these polyprenols influence photosynthetic performance through their modulation of thylakoid membrane dynamics.

Conformational energy calculations have been carried out for three hexadecapeptides in water-namely, a copolymer with the sequence acetyl-AAAAKAAAA-KAAAAKA-amide, 3K(I), in both the charged and neutral forms; a neutral peptide with the sequence acetyl-AAQAAAAQAAAAQAAY-amide, AQY; and a 16-residue L-alanine homopolymer with acetyl and amide terminal groups. The conformational energy was a sum of the empirical conformational energy program for peptides (ECEPP/2) potential energy plus continuum hydration free energy. An empirical (JRF) parameter set was used for the hydration free energy, together with an electrostatic contribution to the solvent effect from charged lysines. The computed relatively high helix content of the most probable conformation of charged 3K(W) and the intermediate helix content of AQY agree reasonably well with experimental values. The computed very low helix content of the alanine homopolymer agrees with experiments on block copolymers and on host-guest random copolymers. The calculations suggest that the hig helix content computed for 3K(I) is due to the sum of internal and hydration free energies of the lysine residues rather than to a high intrinsic helix-forming tendency of alanine. The principal component lowering the computed helix contents of AQY and the alanine copolymer relative to 3K(I) is hydration.Because homopolymers of many naturally occurring amino acids, including L-alanine, are not soluble in water, charged and other highly soluble residues have been incorporated into copolymers in several studies designed to ascertain relative helix stabilities (1-4). In an early approach to determine the helix-forming tendency of L-alanine in water, studies were carried out on the helix-coil transition in (DL-lysine)m-(Lalanine)n-(DL-lysine)m block copolymers (5) of various degrees of polymerization m and n, with n ranging from 10 to 1000. The host-guest technique was used to assess this same tendency in binary random copolymers (6), in which the content of the alanine "guest" ranged from 10 to 50o and the degree of polymerization was from 165 to 1413. These two different types of studies led to values of the Zimm-Bragg parameters (7), oand s, for L-alanine that substantially agreed with each other (6). For example, the value of s was found to be 1.08 at 0C; this value of s would imply a very low helix content for short oligopeptides of L-alanine. In fact, the observed helix content was indistinguishable from zero for a block copolymer having a central block of n = 10 residues (5). A similar value of s for L-alanine was reported by Kemp and colleagues (8), who grew a-helices on a preformed template.In contrast, in a recent experimental study (9) of a 16-residue polypeptide with the sequence acetyl-AAAA-KAAAAKAAAAKA-amide, designated by Marqusee et al. (9) as 3K(I), a helix content of 72% was seen at 10C in 0.01 M NaCl at pH 7.0. This helix content was interpreted in terms of a much larger value (9) of s (-2) for L-alanine at this temperature than that reported by Platzer et al. (6...