Heat-induced synthesis and tunicamycin-sensitive secretion of the putative storage glycoprotein conglutin γ from mature lupin seeds

Authors

  • Marcello DURANTI,

    Corresponding author
    1. Dipartimento di Scienze Molecolari Agroalimentari and Centro Interuniversitario per lo Studio delle Macromolecole Informazionali, Università di Milano, Italy
      Correspondence to M. Duranti, Department of Agrifood Molecular Sciences, University of Milan, Via Celoria 2, I-20133 Milan, Italy
      Fax: +39 2 70633062.
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  • Alessio SCARAFONI,

    1. Dipartimento di Scienze Molecolari Agroalimentari and Centro Interuniversitario per lo Studio delle Macromolecole Informazionali, Università di Milano, Italy
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  • Cristina GIUS,

    1. Dipartimento di Scienze Molecolari Agroalimentari and Centro Interuniversitario per lo Studio delle Macromolecole Informazionali, Università di Milano, Italy
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  • Armando NEGRI,

    1. Istituto di Fisiologia Veterinaria e Biochimica, Università di Milano, Italy
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  • Franco FAORO

    1. Centro Miglioramento Sanitario Colture Agrarie, Consiglio Nazionale delle Ricerche, Milano, Italy
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Correspondence to M. Duranti, Department of Agrifood Molecular Sciences, University of Milan, Via Celoria 2, I-20133 Milan, Italy
Fax: +39 2 70633062.

Abstract

SDS/PAGE, immune blotting with specific antibodies and amino acid sequence analyses revealed that 90% of the protein released from Lupinus albus seeds incubated in water at 60°C for about 3 h was conglutin γ, a putative storage glycoprotein already present in the protein bodies of mature seeds. Incorporation of [14C]leucine into the protein demonstrated that conglutin γ was newly synthesized during the treatment and the use of protein synthesis inhibitors ruled out the secretion of constitutive conglutin γ. Synthesis and secretion took place only over a narrow temperature range, 57.5–62.5°C, and in a short time interval, 135–180 min, of incubation of the seed. The amount of secreted conglutin γ, i.e. 1 mg/seed, was about three times that present inside the treated or untreated seed.

Secreted conglutin γ contained covalently linked carbohydrate as well as the constitutive protein. Inhibition of the glycosylation by tunicamycin did not affect conglutin γ synthesis, but prevented its secretion from the seed, as indicated by quantifying conglutin γ remaining in the seed. An accumulation of the protein outside the protein bodies and at the cotyledonary cell periphery was shown in these samples by immunocytochemistry.

Peptide mapping of the fragments obtained by incubation of constitutive and secreted conglutin γ with trypsin and pepsin revealed no difference between the two proteins.

Lupin seeds were still viable after the treatment. However no similarities between conglutin γ and heat-shock proteins were observed either in the amino acid sequence or other molecular features.

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