def _make_url_prefix(attribute):
    """Make url prefix based on CMIP6 Data Citation Service."""
    # the order of keys is important
    localpart = {
        'mip_era': '',
        'activity_id': '',
        'institution_id': '',
        'source_id': '',
        'experiment_id': '',
    }
    for key, value in attribute:
        if key.localpart in localpart:
            localpart[key.localpart] = value
    url_prefix = '.'.join(localpart.values())
    return url_prefix