The catalytic (C) subunit of cAMP-dependent protein kinase (cAPK) is more stable by several criteria when it is part of a holoenzyme complex. By measuring the thermal stability of the free C subunit in the presence and absence of nucleotides and/or divalent metal ions, it was found that most of the stabilizing effects associated with the type I holoenzyme could be attributed to the nucleotide. The specific requirements for this enhanced stability were further dissected: Adenosine stabilized the C subunit up to 5 degrees C; however, divalent cations (i.e., Mg2+, Ca2+, and Mn2+) do not increase heat stability in combination with adenosine and adenine (1). Divalent cations as well as ATP and ADP have no effect by themselves (2). The enhanced stability derived from both ATP and ADP requires divalent cations. MnATP (12 degrees C) shows a much stronger effect than CaATP (7 degrees C) and MgATP (5 degrees C) (3). In the holoenzyme complex or the protein kinase inhibitor/C subunit complex, metal/ATP is also required for enhanced stability; neither the RI or RII subunits nor PKI alone stabilize the C subunit significantly (4). For high thermal stability, the occupation of the second, low-affinity metal-binding site is necessary (5). From these results, we concluded that the adenine moiety works independently from the metal-binding sites, stabilizing the free C subunit by itself. When the beta- and gamma-phosphates are present, divalent metals are required for positioning these phosphates, and two metals are required to achieve thermostability comparable to adenosine alone. The complex containing two metals is the most stable. A comparison of several conformations of the C subunit derived from different crystal structures is given attributing open and closed forms of the C subunit to less and more thermostable enzymes, respectively.