The plant pathogenesis related proteins group 1 (PR-1) and a variety of related mammalian proteins constitute a PR-1 protein family that share sequence and structural similarities. GAPR-1 is a unique family member as thus far it is the only PR-1 family member that is not co-translationally targeted to the lumen of the endoplasmic reticulum before trafficking to either vacuoles or secretion. Here we report that GAPR-1 may form dimers in vitro and in vivo, as determined by yeast two-hybrid screening, biochemical and biophysical assays. The 1.55A crystal structure demonstrates that GAPR-1 is structurally homologous to the other PR-1 family members previously solved (p14a and Ves V 5). Through an examination of inter-molecular interactions between GAPR-1 molecules in the crystal lattice, we propose a number of the highly conserved amino acid residues of the PR-1 family to be involved in the regulation of dimer formation of GAPR-1 with potential implications for other PR-1 family members. We show that mutagenesis of these conserved amino acid residues leads to a greatly increased dimer population. A recent report suggests that PR-1 family members may exhibit serine protease activity and further examination of the dimer interface of GAPR-1 indicates that a catalytic triad similar to that of serine proteases may be formed across the dimer interface by residues from both molecules within the dimer.