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Identification and characterization of a novel human plant pathogenesis-related protein that localizes to lipid-enriched microdomains in the Golgi complex

¹ Biochemie-Zentrum Heidelberg (BZH), University of Heidelberg, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany
² Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, D-01307 Dresden, Germany
³ Central Spectroscopy Department, German Cancer Research Center, Im Neuenheimer Feld 280, 69120 Heidelberg, Germany
⁴ Max-Planck Institut für Biochemie, Martinsried, Germany

^* Author for correspondence (e-mail: helms{at}uni-hd.de )

Accepted 5 November

Group 1 of plant pathogenesis-related proteins (PR-1) and a variety of related mammalian proteins constitute a superfamily of proteins that share structural similarities. Little is known about their function, but all the family members identified to date are co-translationally translocated to the lumen of the endoplasmic reticulum and are secreted as soluble proteins or are targeted to vacuoles. Here we report the identification of a novel family member that localizes to the cytosolic site of the endomembrane system in mammalian cells. After detergent solubilization of isolated Golgi membranes, a 17 kDa protein was found associated with a low-density detergent-insoluble fraction. The amino-acid sequence, determined by microsequencing and molecular cloning, revealed a significant homology with the superfamily of PR-1 proteins. Golgi-associated PR-1 protein (GAPR-1) showed a brefeldin-A-sensitive Golgi localization in immunofluorescence. Interestingly, the protein remained associated with the microdomain fraction in the presence of Brefeldin A.

By mass spectrometry, GAPR-1 was shown to be myristoylated. Immunoprecipitation of GAPR- 1 from Golgi membranes resulted in the coimmunoprecipitation of caveolin-1, indicating a direct interaction between these two proteins. Myristoylation, together with protein-protein or electrostatic interactions at physiological pH owing to the highly basic pI of GAPR-1 (pI 9.4) could explain the strong membrane association of GAPR-1.

Tissue screening revealed that GAPR-1 is not detectably expressed in liver, heart or adrenal glands. High expression was found in monocytes, leukocytes, lung, spleen and embryonic tissue. Consistent with the involvement of PR-1 proteins in the plant immune system, these data could indicate that GAPR-1 is involved in the immune system.

Key words: Golgi, Microdomains, Rafts, Plant pathogenesis-related protein, PR-1, Myristoylation, Caveolin

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