PhoA/LacZ fusions show AmpP is an inner-membrane protein with 10 TM helices.
"AmpP fusions at amino acids 80, 170, 248, 308, 400 as well as full length were LacZ-positive and PhoA-negative, and fusions at amino acids 38, 120, 195, 278, and 360 were LacZ-negative and PhoA-positive, consistent with 10 TM domains (Figures 4A and 4B)."