Primary extracellular structural protein

Fig. 5. Hypothetical scheme for the evolution of heterothallism and secondary homothallism in Mycobionta. For synchronous nuclear division in Benjaminiella multispora, Cokeromyces and Mucor species with respect to primary homothallism, compare Forst and Prillinger [159]. For molecular details, see Hiscock and Kiies [90 and the literature cited therein]. For secondary homothallsim, see Glass et al. [160] and Yun et al. [95]. IC = Intracellular function EC = extracellular function SP = structural proteins which are involved in pheromone binding RP = regulatory proteins which are involved in n-DNA binding and regulation of transcription.

The hydroxy-L-proline-rich protein present in the culture medium of suspension-cultured sycamore-cells was shown to be covalently linked to an arabinogalactan,57 and this led to speculation regarding the existence of a similar interconnection within the cell wall. However, it has now been established that the hydroxy-L-proline-rich protein of the primary wall is structurally different from that present in the extracellular fluid234 (see Section VII, 1), and thus the latter cannot be used as a model for the cell-wall glycoprotein. 

The primary level of structure in a protein is the linear sequence of amino acids as joined together by peptide bonds. This sequence is determined by the sequence of nucleotide bases in the gene encoding the protein (see Topic HI). Also included under primary structure is the location of any other covalent bonds. These are primarily disulfide bonds between cysteine residues that are adjacent in space but not in the linear amino acid sequence. These covalent cross-links between separate polypeptide chains or between different parts of the same chain are formed by the oxidation of the SH groups on cysteine residues that are juxtaposed in space (Fig. 4). The resulting disulfide is called a cystine residue. Disulfide bonds are often present in extracellular proteins, but are rarely found in intracellular proteins. Some proteins, such as collagen, have covalent cross-links formed between the side-chains of Lys residues (see Topic B5). 

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