Proteinase K stock:

After doing an extensive search of the web, I found that Ambion offers proteinase K dissolved in 50 mM Tris pH 8, 3 mM CaCl2, 50% glycerol.  To make a stock from powder, dissolve the proteinase K at 20 mg/ml in 100 mM Tris pH 8, 6 mM CaCl2 and then dilute with an equal volume of 100% glycerol.  Store the Proteinase K at -20.

It turns out that proteinase K activity is stimulated by the presence of CaCl2; hence, CaCl2 is in the storage buffer.