Distribution of CAAX proteases and associated AAXing activity by subcellular fractionation. (A) A total yeast lysate derived from a strain expressing HA-Ste24p and Kex2p-HA was fractionated on a sucrose step gradient. Equivalent volumes from each fraction (50 μl) were solubilized in Laemmli’s sample buffer, subjected to SDS/15% PAGE, transferred to nitrocellulose, and probed with antibodies for Rce1p, HA (HA-Ste24p and Kex2p-HA), and organellar markers. Gradient fractions were also assayed for protein concentration and AAXing activity, as described in Materials and Methods. The broken line represents an arbitrary division between light and heavy membranes. The strain used is SM3365/pSM962/pSN218/pSM1153. (B) Total membranes prepared from yeast cells of the indicated genotypes were assayed for AAXing activity, as above. Strains used are (left to right) SM1058/pRS316, SM3103/pRS316, SM3613/pRS316, and SM3614/pRS316.