The limited proteolysis procedure was optimized from standard protocols and previous experiments . 1 mg/ml stocks were prepared from 25 mg of lyophilized Proteinase K (PK) were dissolved in storage buffer (50 mM Tris-HCl, 2 mM calcium acetate, pH 8.0) and stored at -70 °C. The following concentrations of PK were prepared from serial dilutions from the 1 mg/ml aliquot: 0.5 mg/ml, 0.2 mg/ml, 0.1 mg/ml, 0.05 mg/ml, and added to lysate to yield 1:200, 1:500, 1:1000, and 1:2000 protease:substrate protein ratios (w/w) respectively. 2 µl PK was added to a 200µg aliquot of protein lysate and incubated for 1 min at 25.0 °C for each digestion.