The protein sample was packed into a 1.3 mm rotor using centrifugation at 10,000 relative centrifugal force in a benchtop centrifuge. 0.2 l of a saturated solution of DSS in sample buffer was added to the protein sample before closing the rotor to allow for internal referencing. A thin layer of glue was added to seal the caps (see e.g ref for a detailed description of the rotor packing procedure). All solid state NMR experiments were recorded on a 600 MHz spectrometer (Bruker) equipped with a triple channel (HCN) 1.3 mm probe (Bruker) operating at a MAS rate of 55 kHz. C detected 2D hnCACX spectra were recorded on MLF using a double sensitivity-enhanced hNCACX pulse sequence modified from a previously published hNCACO pulse sequence 2 . Cross-polarization (CP) was used for 1 H- N transfers, SPEPS 3 for 15 N- C transfers and TOCSY (DIPSI-3) , TOBSY (C948 1 ) or hSPEPS for 13 C- CX transfers. Experiments on GlpG were performed at a sample temperature of ca 20 °C, based on the chemical shift of the water peak referenced internally to DSS. Standard and sensitivity-enhanced C detected 2D hCC spectra were recorded using the different mixing sequences mentioned above. 3D and 4D experiments were recorded using sensitivityenhanced pulse sequences and 1 H detection. CP was used for the first magnetization transfer step ( 1 H- C, 1 H- CO or 1 H-13 CX). TROP shapes specific for 55 kHz MAS were used for C-15 N, CO-15 N, 13 C-13 CO (specific for 600 MHz spectrometer), CO-13 C (specific for 600 MHz spectrometer) and 15 N-1 H transfers. DIPSI-3 was used for CX- C transfers. 3D hCANH and hCONH spectra were recorded in a uniform fashion and processed using TopSpin v4 (Bruker). 4D spectra were recorded using 5 % non-uniform sampling (NUS).Two additional 3D hCXcaNH spectra, one with TOCSY mixing and one with hSPEPS mixing, were recorded using 25% NUS. 10 kHz Waltz-64 decoupling was applied to the 1 H channel during acquisition of the indirect dimensions and on the 15 N and C channels during direct 1 H acquisition. MISSISSIPPI was used for water suppression. Echo-Antiecho was used for all indirect dimensions in sensitivity-enhanced experiments and States-TPPI was used in standard experiments. All sensitivity-enhanced pulse sequences were modified based on previously published sensitivity-enhanced pulse sequences to allow for additional dimensions to be recorded and to introduce 13 C- C mixing for side-chain assignments. A scheme of the sensitivityenhanced hCXCANH pulse sequence is provided in SI Fig. . Original pulse sequences and TROP shapes were downloaded from www.optimal-nmr.net . NUS lists were generated from . NUS spectra were reconstructed and processed using nmrPipe . Experimental details for all assignment spectra are provided in SI tables 1-2. Processing details for NUS spectra are provided in SI table 3 and details for the conversion from Bruker to nmrPipe format is shown in SI figure . CCPNMR AnalysisAssign v3.1 was used for data analysis, assignments and to generate figures of multidimensional NMR spectra.