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HA-MOP knockin mice express the canonical µ-opioid receptor but lack detectable splice variants

ABSTRACT: G protein-coupled receptors (GPCRs) are notoriously difficult to detect in native tissues. In an effort to resolve this problem, we have developed a novel mouse model by fusing the hemagglutinin (HA)-epitope tag sequence to the amino-terminus of the µ-opioid receptor (MOP). Although HA-MOP knock-in mice exhibit reduced receptor expression, we found that this approach allowed for highly efficient immunodetection of low abundant GPCR targets. We also show that the HA-tag facilitates both high-resolution imaging and immunoisolation of MOP. Mass spectrometry (MS) confirmed post-translational modifications, most notably agonist-selective phosphorylation of carboxyl-terminal serine and threonine residues. MS also unequivocally identified the carboxyl-terminal 387LENLEAETAPLP398 motif, which is part of the canonical MOP sequence. Unexpectedly, MS analysis of brain lysates failed to detect any of the 15 MOP isoforms that have been proposed to arise from alternative splicing of the MOP carboxyl-terminus. For quantitative analysis, we performed multiple successive rounds of immunodepletion using the well-characterized rabbit monoclonal antibody UMB-3 that selectively detects the 387LENLEAETAPLP398 motif. We found that >98% of HA-tagged MOP contain the UMB-3 epitope indicating that virtually all MOP expressed in the mouse brain exhibit the canonical amino acid sequence.

INSTRUMENT(S): LTQ Orbitrap Velos

ORGANISM(S): Mus Musculus (mouse)

TISSUE(S): Brain

SUBMITTER: Carine Froment

LAB HEAD: Stefan Schulz

PROVIDER: PXD027825 | Pride | 2022-12-07

REPOSITORIES: Pride

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Project description:CarD is an essential mycobacterial protein that we had previously shown to bind the RNA polymerase (RNAP) and affect the transcriptional profile of M. smegmatis and Mycobacterium tuberculosis. For this reason, we suspected that CarD was directly regulating transcriptional complexes but we did not know at what stage of CarD was functioning and at which genes CarD interacted with the RNAP. To determine in which stage of the transcription cycle (initiation, elongation, or termination) CarD acts, we used Chromatin Immunoprecipitation sequencing (ChIP-seq) to survey the distribution of CarD throughout the M. smegmatis chromosome. Specific antibodies targeting core RNAPb, RNAPσ, or a hemagglutinin (HA) epitope fused to CarD (CarD-HA) were used to co-immunoprecipitate associated DNA. CarD-HA was immunoprecipitated from the M. smegmatis Mc2155 ΔcarD attB::tetcarD-HA strain and unfused HA was immunoprecipitated from the Mc2155 attB::pmsg431 strain with monoclonal antibodies specific for HA (Sigma). RNAP β and σ were immunoprecipitated from M. smegmatis ΔcarD attB::tetcarD-HA with monoclonal antibodies specific for these subunits (Neoclone, Madison, WI; 8RB13 for β, 2G10 for σ). Co-precipitated DNA was sequenced using a SOLiD sequencer (Life Technologies), which provided sufficient reads for 100-fold coverage of the genome. The number of sequence reads per base pair was normalized to the total number of reads and expressed as a log2 value. The reads per base pair from the HA-alone sample served as the background and was subtracted from the other datasets. We found that CarD was never present on the genome in the absence of RNAP. However, whereas RNAP core enzyme was found throughout transcribed regions of the genome, CarD was primarily associated with promoter regions and highly correlated with RNAPσ. The colocalization of σA and CarD led us to propose that in vivo, CarD associates with RNAP initiation complexes at most promoters and is therefore a global regulator of transcription initiation. The genome sequences associated with M. smegmatis CarD, RNAPb, and RNAPs were determined by ChIP-seq analysis. Samples were done in duplicate, except for RNAPs. And sequencing was performed using a SOLiD sequencer (Life Technologies).

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HA-MOP knockin mice express the canonical µ-opioid receptor but lack detectable splice variants

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