Synthesis of Disease-Associated Proteins in Viroid-Infected Tomato Leaves and Binding of Viroid to Host Proteins

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Abstract
Comparison of protein synthesis from uninfected and potato spindle tuber viroid (PSTV)-infected tomato leaves showed that three proteins of 14, 33, and 90-100 kDa were enhanced in infected tissue. The 33 kDa protein was the most prominent. These proteins were distributed in soluble subcellular fractions. A 33 kDa protein that bound PSTV in vitro was present at detectable levels in the soluble fractions from infected but not uninfected cells. Both the infected and uninfected fractions also contained PSTV-binding proteins of 52-55, 35, 31-32, and 16-20 kDa. With the exception of the 35 and 33 kDa proteins, all the other PSTV-binding proteins also were detected in in vivo complexes of PSTV with subcellular constituents obtained by density-gradient fractionation of 20,000-70,000 g pellet fractions from infected cells. The PSTV-binding proteins in the complexes are of host origin. Treatment of the purified PSTV-cellular constituent complexes with pronase or triton-X-100 released PSTV; thus PSTV appears to be noncovalently bound to proteins in these complexes.