Cleavage of Disulfide-Bridged Stalk Domains during Shedding of Angiotensin-Converting Enzyme Occurs at Multiple Juxtamembrane Sites

Abstract

Shedding of the ectodomain of angiotensin-converting enzyme (ACE) and numerous other membrane-anchored proteins results from a specific cleavage in the juxtamembrane (JM) stalk, catalyzed by "sheddases" that are commonly activated by phorbol esters and inhibited by peptide hydroxamates such as TAPI. Sheddases require a stalk of minimum length and steric accessibility. However, we recently found that substitution of the ACE stalk with an epidermal growth factor (EGF)-like domain from the low-density lipoprotein receptor (LDL-R) did not abolish shedding; cleavage of the ACE-JMEGF chimera occurred at a Gly-Phe bond in the third disulfide loop of the EGF domain. We have now constructed two additional stalk chimeras, in which the native stalk in ACE was replaced with the EGF domain from factor IX (ACE-JMfIX) and with a cysteine knot motif (ACE-JMmin23). Like the ACE-JMEGF chimera, the ACE-JMfIX and -JMmin23 chimeras were also shed, but mass spectral analysis revealed that the cleavage sites were adjacent to, rather than within, the disulfide-bonded domains. Homology modeling of the LDL-R EGF domain revealed that the third disulfide loop is larger and more flexible than the equivalent loop in the factor IX EGF domain. Similarly, the NMR structure of the Min-23 motif is highly compact. Hence, cleavage within a disulfide-bonded domain appears to require an unhindered loop. Interestingly, unlike wild-type ACE and the ACE-JMEGF and -JMmin23 chimeras, shedding of the ACE-JMfIX chimera was not stimulated by phorbol or inhibited by TAPI, but instead was inhibited by 3,4-dichloroisocoumarin, indicating the activity of an alternative sheddase. In summary, the ACE shedding machinery is highly versatile, but an accessible JM sequence, in the form of a flexible stalk or an exposed loop within or adjacent to a folded domain, appears to be required. Moreover, alternative sheddases are recruited, depending on the nature of the JM sequence.