Activated T cells cultured in the absence of l-Arg have a decreased Rb phosphorylation and a decreased ability to phosphorylate Rb in vitro. (A) Whole-cell lysates obtained from stimulated T cells cultured for 24 hours in the presence or the absence of l-Arg were immunoblotted against Rb and vinculin. (B) Lysates from unstimulated or stimulated T cells cultured for 24 hours in the presence or the absence of l-Arg were prepared, and 100 μg was immunoprecipitated with agarose-conjugated anti–cyclin D3 and incubated with a reaction cocktail containing 0.2 μg GST-Rb and 10 μCi (0.37 MBq) [γ-³²P] ATP. (C) Similarly, 100 μg whole-cell lysates from stimulated T cells cultured for 24 hours in medium containing l-Arg was initially immunodepleted with an irrelevant protein G (NRS), or agarose-conjugated antibodies against cyclin D3, cdk4, or cdk6, and then immunoprecipitated with irrelevant protein G (NRS) or anti–cyclin D3 and tested for kinase activity.